RFR: 7834: Improve GitHub Actions validation workflow

[Christoph Langer]

Improve validation workflow.

- It should not run on "pull_request" (OpenJDK repos don't allow this but will transfer results of push runs in forked repositories into status checks of PRs)
- A possibility to trigger the workflow manually via "workflow_dispatch" would be nice
- Updates to a branch should cancel running actions in favor of a new run to save resources
- The workflow script can have less redundant code by using a matrix strategy for the different operating systems that are tested
- Used actions can be version-bumped
- Naming of some steps can be improved
- The test steps and their order can be improved to make the steps more logical and explicit:
  1. Build&Test core
  2. Start P2 repository
  3. Build&Test application
  4. Build&Test agent
  5. Run spotless
- Use maven option --no-transfer-progress to get less noisy logs

Now also the Windows actions run in the bash shell and use the .sh scripts instead of their .bat counterparts.
One could think of deleting them but maybe they are used elsewhere, so keep them (up to date).

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Commit messages:
 - Improve validation workflow

Changes: https://git.openjdk.org/jmc/pull/407/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=407&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7834
  Stats: 181 lines in 11 files changed: 53 ins; 93 del; 35 mod
  Patch: https://git.openjdk.org/jmc/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/407/head:pull/407

PR: https://git.openjdk.org/jmc/pull/407