Add CI predefined environment variable CI_COMMIT_TIMESTAMP

What does this MR do?

Adds a new Predefine Environment Variable CI_COMMIT_TIMESTAMP variable to the pipeline. The variable value is represented as ISO-8601 date-time format.

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

Will it require cross-browser testing? No

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

Closes #34281 (closed)