Only validate gitaly timeouts when they changed

What does this MR do?

This avoids validating Gitaly timeouts, that could have become invalid based on configuration in omnibus.

The invalid values would not be used, instead the mininum value from omnibus would be used. But this would prevent the application settings from being updated.

Skipping these validations makes sure the settings can still be saved if unrelated values are changed.

This also adds some documentation for these timeouts, so administrators have an idea what these values should be.

Closes #208432 (closed)

Follow up from !21492 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done