ICU-22723 Cleanup: remove the icu4j/maven-migration/ folder

[mihnita]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22723
• Required: The PR title must be prefixed with a JIRA Issue number.
• 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.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[mihnita]

I hope that the issues is OK (ICU-22723: "ICU 76rc BRS")
ICU-22722 "ICU 76 docs minor fixes" didn't seem like a better match.

[echeran]

LGTM

[markusicu]

rc BRS ticket is fine