Drop EL7 (release-4.1)

[dtrudg]

Description of the Pull Request (PR):

Backport various changes made on the main branch, related to end of life of EL7, and removal of test dependencies on CentOS 7.

Covers:

• Removal of EL7 CI job.
• Removal of now-broken CentOS7 dependent tests and examples.
• Removal of CentOS7 / EL7 content from markdown files.

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR, linted with make check and tested this PR locally with a make test, and make testall if possible (see CONTRIBUTING.md).
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File