ICU-22883 Update CLDR-ICU instructions with CLDR prereqs for BRS integrations

[pedberg-icu]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22883
• 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

The CLDR-ICU integration instructions should mention that for CLDR-ICU integrations that are part of the BRS steps for CLDR and ICU, there are some prerequisites on the CLDR side:

1. The CLDR task to update test data should be run)
2. THE CLDRModify passes no-options and -fP should be run