chore: set explicit edition in .rustfmt.toml

[SimonThormeyer]

This results in requiring less manual setup.

What's new in this PR

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PR Submission Checklist for internal contributors

• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Bencher Report

Branch	simon/chore/rustfmt-edition
Testbed	ubuntu-latest

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Benchmark	Latency	microseconds (µs)
Commit add f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	18,452.00 µs
Commit add f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	777.40 µs
Commit add f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	4,429.30 µs
Commit add f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	8,359.10 µs
Commit add f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	12,104.00 µs
Commit add f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	15,018.00 µs
Commit add f(number clients)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	1,101,600.00 µs
Commit add f(number clients)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	765.47 µs
Commit add f(number clients)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	88,035.00 µs
Commit add f(number clients)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	241,180.00 µs
Commit add f(number clients)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	473,540.00 µs
Commit add f(number clients)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	751,280.00 µs
Commit pending proposals f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	134,090.00 µs
Commit pending proposals f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	25,792.00 µs
Commit pending proposals f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	47,831.00 µs
Commit pending proposals f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	65,972.00 µs
Commit pending proposals f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	88,211.00 µs
Commit pending proposals f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	106,670.00 µs
Commit pending proposals f(pending size)/cs1/mem/1	📈 view plot ⚠️ NO THRESHOLD	17,888.00 µs
Commit pending proposals f(pending size)/cs1/mem/101	📈 view plot ⚠️ NO THRESHOLD	132,240.00 µs
Commit pending proposals f(pending size)/cs1/mem/21	📈 view plot ⚠️ NO THRESHOLD	38,282.00 µs
Commit pending proposals f(pending size)/cs1/mem/41	📈 view plot ⚠️ NO THRESHOLD	63,274.00 µs
Commit pending proposals f(pending size)/cs1/mem/61	📈 view plot ⚠️ NO THRESHOLD	85,671.00 µs
Commit pending proposals f(pending size)/cs1/mem/81	📈 view plot ⚠️ NO THRESHOLD	108,010.00 µs
Commit remove f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	10,513.00 µs
Commit remove f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	581.37 µs
Commit remove f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	2,189.70 µs
Commit remove f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	3,903.70 µs
Commit remove f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	5,982.20 µs
Commit remove f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	7,808.90 µs
Commit remove f(number clients)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	13,642.00 µs
Commit remove f(number clients)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	150,000.00 µs
Commit remove f(number clients)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	122,920.00 µs
Commit remove f(number clients)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	95,899.00 µs
Commit remove f(number clients)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	68,337.00 µs
Commit remove f(number clients)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	40,949.00 µs
Commit update f(group size)/cs1/mem/1002	📈 view plot ⚠️ NO THRESHOLD	149,720.00 µs
Commit update f(group size)/cs1/mem/2	📈 view plot ⚠️ NO THRESHOLD	780.48 µs
Commit update f(group size)/cs1/mem/202	📈 view plot ⚠️ NO THRESHOLD	31,090.00 µs
Commit update f(group size)/cs1/mem/402	📈 view plot ⚠️ NO THRESHOLD	60,909.00 µs
Commit update f(group size)/cs1/mem/602	📈 view plot ⚠️ NO THRESHOLD	91,720.00 µs
Commit update f(group size)/cs1/mem/802	📈 view plot ⚠️ NO THRESHOLD	120,750.00 µs

🐰 View full continuous benchmarking report in Bencher