Reduce frequency of image testing

### Context
As of today, we're testing the whole Support Matrix every night.
In other teams, eg EA they're testing it once https://github.com/elastic/ingest-dev/issues/1326 or for the next Oracle 10 image https://github.com/elastic/ingest-dev/issues/5767

Our daily executions schedule:
- Adds burden if they fail, for various reasons. Eg the last failure was due to unavaliability of the AMI in the cloud providers https://github.com/elastic/apm-server/issues/20463 and we need to find a solution here https://github.com/elastic/apm-server/issues/20561 (being discussed with PMs)
- Adds costs for low ROI

### Goal
Reduce frequency of testing, to be decided:

- Run tests automatically (as of today) but once every X weeks
- Run tests manually when we have a release, and introduce this as part of the release manager duty