Fall back to previous minor, if necessary

[seanstory]

Related to https://github.com/elastic/search-team/issues/11615

#3892 CI is currently failing, like:

+ echo 'Failed to get tarball URL for version 9.4.0-SNAPSHOT, trying next...'
+ continue
+ echo 'Error: Failed to fetch any compatible Elasticsearch version'
+ return 1

This is happening because there's no PATCH before 0 in the 9.4.0 current version to fall back to. But I don't like having to wait for Elasticsearch to bump to a new minor before we can bump our branches every time.

This change adds an undocumented Env Var that will allow our Ftests to bypass version compatibility checks, and also lets the ftest shell script download a past MINOR version of ES, if it can't find one for the current version AND if the current patch version is 0. This scenario should only occur when we are all bumping to a new minor.

Checklists

Pre-Review Checklist

• this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• this PR has a meaningful title
• this PR links to all relevant github issues that it fixes or partially addresses
• this PR has a thorough description
• Covered the changes with automated tests
• Tested the changes locally
• Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)

[Apmats]

We do frequently go beyond 5 in patches, but it shouldn't matter much here as we usually don't have x.x.6 before we have x.x+1.0 out?

[seanstory]

Yeah exactly. I originally had 9 8 7 6 5 4 3 2 1, but this seemed excessive, and each one adds a little latency.

[Apmats]

Nice workaround, and thank you.

[github-actions]

💔 Failed to create backport PR(s)

Status	Branch	Result
❌	9.4	The branch "9.4" is invalid or doesn't exist

To backport manually run:
backport --pr 3893 --autoMerge --autoMergeMethod squash