ICU-22921 fix link from gitdev to ci exhaustive tests

[markusicu]

https://unicode-org.github.io/icu/userguide/dev/gitdev.html#requesting-an-exhaustive-test-run-on-a-pull-request-pr points to Continuous Integration / Exhaustive Tests but has a bad indirection. It should just go sideways to its sibling page.

Checklist

• Required: Issue filed: ICU-22921
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable