Test integrations with latest stack version on PRs

Currently all integrations are tested in PRs with the oldest version they support. This helps avoiding the introduction of breaking changes, but it relies on developers to test with recent versions.

We could add a new testing stage for PRs, that tests changes with an snapshot version.

The changes would be:
* Add a stage that tests on PRs with an snapshot version of the stack, apart of the current stage that test with the oldest supported version.
* Add an automation to open a PR to update the snapshot version used. Failures here should follow same strategy as https://github.com/elastic/integrations/issues/6071. This automation could replace the daily job for latest snapshot.

This will basically duplicate the resources used to test each PR.

Related to https://github.com/elastic/integrations/issues/213