ICU-21615 Work-around Azure CI build bot issue on Ubuntu 18.04

[jefgen]

There was a recent update the Azure CI Ubuntu 18.04 build bot image which causes the C++14 Debug build bot to fail in the tests with a segfault.

We initially thought this was related to ICU-changes, but it turns out that it was a change to the build bot image itself.

Using the work-around from this thread to remove GCC 11 fixes the issue.
See: actions/runner-images#3376

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21615
• 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