Fix GitLab commit date parsing across cultures

[SakeruW]

Fixes #1626

• Did you write/update appropriate tests - added culture and offset cases.
• Release notes updated (if appropriate) - updated RELEASENOTES.md.
• Appropriate logging output - no logging change is needed for this parsing fix.
• Issue linked - fixes gl2gh inventory-report failed to parse commit dates under en-SG region #1626.
• Docs updated (or issue created) - no documentation change is needed for this internal parsing fix.
• New package licenses are added to ThirdPartyNotices.txt (if applicable) - not applicable. no packages were added.

Description

gl2gh inventory-report could fail or misread GitLab commit dates under some regional cultures.
This change preserves the API’s ISO 8601 timestamp and parses it with invariant culture, with regression tests for culture and offset handling.

[github-actions]

Unit Test Results

    1 files      1 suites   24s ⏱️
1 184 tests 1 184 ✅ 0 💤 0 ❌
1 185 runs  1 185 ✅ 0 💤 0 ❌

Results for commit d632be7.

♻️ This comment has been updated with latest results.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
gl2gh	78%	71%	420	✔
gei	81%	74%	688	✔
Octoshift	81%	71%	2046	✔
bbs2gh	83%	78%	669	✔
ado2gh	71%	70%	749	➖
Summary	79% (9063 / 11424)	72% (2323 / 3219)	4572	✔