ICU-22722 Remove old announcement of default git branch name

[echeran]

Checklist

• [X] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22722
• [X] Required: The PR title must be prefixed with a JIRA Issue number.
• [X] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [X] Required: Each commit message must be prefixed with a JIRA Issue number.
• [X] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [X] API docs and/or User Guide docs changed or added, if applicable