remove github.com/phayes/permbits dependency

[thaJeztah]

It's trivial to implement an equivalent, and was only used in tests.

- Description for the changelog

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.70%. Comparing base (6e9e7b8) to head (013758d).
⚠️ Report is 60 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3282      +/-   ##
==========================================
- Coverage   14.73%   14.70%   -0.03%     
==========================================
  Files         200      200              
  Lines       93077    93027      -50     
==========================================
- Hits        13712    13680      -32     
+ Misses      78019    78000      -19     
- Partials     1346     1347       +1     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR removes the github.com/phayes/permbits dependency from the main swarmkit/v2 module and vendors, replacing its limited test usage with small local permission-bit helpers.

Changes:

• Drop github.com/phayes/permbits from go.mod, go.sum, and vendor/modules.txt.
• Remove the vendored vendor/github.com/phayes/permbits package.
• Update ca/certificates_test.go to assert permissions via os.Stat + local bit checks instead of permbits.Stat.

Reviewed changes

Copilot reviewed 2 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
vendor/modules.txt	Removes the permbits module entry from vendored module metadata.
vendor/github.com/phayes/permbits/README.md	Deletes vendored permbits README as part of dependency removal.
vendor/github.com/phayes/permbits/permbits.go	Deletes vendored permbits implementation.
vendor/github.com/phayes/permbits/LICENSE	Deletes vendored permbits license file.
vendor/github.com/phayes/permbits/godoc.go	Deletes vendored permbits package docs file.
vendor/github.com/phayes/permbits/.travis.yml	Deletes vendored permbits CI config file.
go.sum	Removes permbits checksums from the main module dependency graph.
go.mod	Removes the direct permbits requirement from the main module.
ca/certificates_test.go	Replaces permbits-based permission assertions with local helpers using os.FileMode.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.