Add .config/ to .gitignore

[kaovilai]

Thank you for contributing to Velero!

Please add a summary of your change

Add .config/ directory to .gitignore with detailed explanation of why this directory gets created during builds.

The .config/ directory is created by Go's telemetry system when the Makefile uses docker run -w /github.com/vmware-tanzu/velero. This sets the container's HOME environment variable to the working directory, causing Go's os.UserConfigDir() to fall back to $HOME/.config when XDG_CONFIG_HOME is unset, creating .config/ in the project root.

Does your change fix a particular issue?

This prevents the .config/ directory (created during builds) from appearing as an untracked file in git status.

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.84%. Comparing base (f8b799d) to head (90d13bb).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9006      +/-   ##
==========================================
+ Coverage   59.61%   59.84%   +0.22%     
==========================================
  Files         374      375       +1     
  Lines       40726    41046     +320     
==========================================
+ Hits        24280    24563     +283     
- Misses      14963    14996      +33     
- Partials     1483     1487       +4     

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