ICU-22533 Don't mention 'Release Candidate' in javadoc

[mihnita]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22533
• 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

[markusicu]

Ok, but I think we wanted to retain the language as something that we would want to do again in the future if & when we automate the API docs generation further. If we don't retain that here, then where?

[mihnita]

Should I open a bug?

[markusicu]

Should I open a bug?

Yes, please.