Use a fixed version of core in tests and automate updates to it

[timja]

What feature do you want to see added?

I would like to get a PR created by updatecli / whatever that bumps the version of core that ATH tests against, currently it just always runs against the last weekly but that means we don't recognise regressions when they happen and there's no notification to check the test suite.

In bom we do:

https://github.com/jenkinsci/bom/blob/master/.github/workflows/updatecli.yaml https://github.com/jenkinsci/bom/blob/master/updatecli/updatecli.d/weekly-apply.sh

Currently the version is set in https://github.com/jenkinsci/acceptance-test-harness/blob/master/src/main/resources/ath-container/run.sh#L48 where it just uses latest and

I think what we want to do is change the Jenkinsfile to read a file with the version number we're testing against.

The reason to use a file is to allow 'untrusted' contributors to test against different versions without needing write access

Upstream changes

No response

[jtnord]

semi-related #744