feat: Add CI_GITLAB_FIPS_MODE to predefined CI variables

What does this MR do and why?

Adds new predefined pipeline variable that exposes whether or not the instance's FIPS-mode is enabled. This is to be used to provide conditional logic within pipeline templates and devopssecure analyzers directly

Relates to #353523 (closed)

Changelog: added

How to set up and validate locally

1. Enable FIPS-mode feature flag
2. Run pipeline outputting CI_GITLAB_FIPS_MODE

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