fix: JMAP update spec URLs

[toddheasley]

Contribution Summary

jmap.io changed URL format w/ no redirect for old links.

Close #376

AI Contribution Disclosure

• This contribution does not include any changes created or assisted by AI.
• This contribution includes changes assisted by AI.
• This contribution includes changes created by AI.

Contribution Checklist

• I have read and affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution adheres to the Thunderbird iOS Contribution Guidelines.
• This contribution does not contain merge commits, and is rebased on the latest from the iOS codebase.
• This contribution includes tests for any new functionality, and maintains tests for any updated functionality.
• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).