Fix is_valid_timestamp for strings with timezone

[g-marconet]

Description

• Timestamp format was %Y-%m-%dT%H:%M:%SZ
  • This works fine with times in UTC (with a "Z" at the end), but doesn't parse non-UTC timezones
• Gitlab seems to not use UTC. This example was taken from their docs: 2024-06-28T03:44:20-07:00
  • This would fail with the previous timestamp format

Motivation and Context

Previously, this bug would cause issues retrieving timestamp information for Gitlab.

How Has This Been Tested?

Tested in dev environment and by crafting strings

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist

• My code follows conforms to the coding standards.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[ZoogieZork]

Just need a documentation update.