ICU-22977 Rollback to windows-2019 temporarily to unblock CI

[rp9-next]

This change rollbacks to previous version of windows VM runner temporarily to unblock devs from actions/runner-images#11016

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22977
• 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

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• .github/workflows/icu_merge_ci.yml is now changed in the branch
• .github/workflows/icu4c.yml is different
• icu4c/source/common/common_uwp.vcxproj is no longer changed in the branch
• icu4c/source/i18n/i18n_uwp.vcxproj is no longer changed in the branch

View Diff Across Force-Push

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