Removing gitlab header for completeness after GA

[brianaj]

This pull request makes a small change to the GithubClient class to update the default HTTP request headers. Specifically, it removes the octoshift_ll_gitlab_self_serve feature from the GraphQL-Features header.

• Did you write/update appropriate tests
• Release notes updated (if appropriate)
• Appropriate logging output
• Issue linked
• Docs updated (or issue created)
• New package licenses are added to ThirdPartyNotices.txt (if applicable)

[github-actions]

Unit Test Results

    1 files      1 suites   26s ⏱️
1 152 tests 1 152 ✅ 0 💤 0 ❌
1 153 runs  1 153 ✅ 0 💤 0 ❌

Results for commit aac3849.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
gei	81%	74%	682	✔
Octoshift	80%	70%	1993	✔
gl2gh	77%	70%	417	✔
bbs2gh	83%	78%	668	✔
ado2gh	71%	70%	749	➖
Summary	79% (8901 / 11275)	72% (2275 / 3165)	4509	✔