ICU-22532 Use previous Ubuntu version for ICU4C in GH Actions CI

[echeran]

Workaround for an ephemeral issue of ./runConfigureICU failing for a few CI jobs in Github Actions for ICU4C, seemingly caused by the underlying GHA runner VM's software changing someone in the latest version. (Runner VM version 20240304.1.0 seemed fine, but 20240310.1.0 seems to be a problem. Explains almost all of our observed behavior, modulo a few exceptions.)

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22532
• 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