dependabot: add dependabot file

[storopoli]

Description

Adds dependabot configuration with cooldown set to 14 days.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature/Enhancement (non-breaking change which adds functionality or enhances an existing one)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor
• New or updated tests
• Dependency Update

Notes to Reviewers

Discussed with the security team at Alpen Labs.
Removes the dependencies.yml CI job which is now being done by dependabot.

Checklist

• I have performed a self-review of my code.
• I have commented my code where necessary.
• I have updated the documentation if needed.
• My changes do not introduce new warnings.
• I have added (where necessary) tests that prove my changes are effective or that my feature works.
• New and existing tests pass with my changes.
• I have disclosed my use of AI in the body of this PR.

Related Issues

STR-1827

[github-actions]

Commit: fe7d603

SP1 Execution Results

program	cycles	success
fibonacci	1,189	✅
sha2_chain	8,032	✅
schnorr_sig_verify	4,435,807	✅
groth16_verify_sp1	10,363,813	✅
groth16_verify_risc0	162,795,473	✅

RISC0 Execution Results

program	cycles	success
fibonacci	3,694	✅
sha2_chain	9,894	✅
schnorr_sig_verify	4,470,687	✅
groth16_verify_sp1	49,049,811	✅
groth16_verify_risc0	163,016,787	✅