Backport fix case insensitivity in codeowners

What does this MR do and why?

Currently, CODEOWNERS validation incorrectly rejects valid usernames and group names when the case doesn't exactly match. For example, @REPORTED_user_21 would be rejected even if the user exists as @reported_user_21 in GitLab's database.

This creates confusion for users who expect case-insensitive matching, similar to how GitLab handles usernames and group paths elsewhere in the application.

Related to: CODEOWNERS Syntax Check is Case Sensitive, Appr... (#553323 - closed) The original MR: Fix case sensitivity in CODEOWNERS validation (!200043 - merged)

This MR fixes case sensitivity issues in CODEOWNERS validation by making username and group name matching case-insensitive.

Actual usernames in GitLab: @reported_user_21, @TestUser, group path: @codeowner-test1

Before: @REPORTED_user_21 returns a syntax error	After: @REPORTED_user_21, @testUser, @Codeowner-TEST1 all passed

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.