fix: Omit invalid chat input field for GitLab 17.2 and earlier

Description

Implement a cached instance version check to avoid sending non-existent fields to GitLab 17.2 and earlier.

Related Issues

Resolves Unable to send chat messages in Self-Managed af... (#1471 - closed).

How has this been tested?

• New test coverage.

I'm planning on running these additional smoke tests before asking for this change to be released:

• Ran the plugin locally against GitLab 17.3 and later (e.g. gitlab.com)
• Ran the plugin pointing to GitLab 17.2 and earlier (mock token server)

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes

Closes #1471 (closed)