Refactor: Consolidate CI jobs using matrix strategies (#9311)

Consolidates the GitHub Actions workflow by using matrix strategies for JVM and provider tests.

- Replaces separate `testopenjdk*` jobs with a single `test_jvm` job that runs tests against Java versions 8, 11, 17, and 21 using a matrix.
- Replaces separate provider test jobs (`testopenjsse`, `testconscrypt`, `testbouncycastle`, `testcorretto`) with a single `test_providers` job that uses a matrix for different security providers.
- Adds workflow concurrency to cancel in-progress runs on the same branch.
- Removes redundant JDK setup steps and cleans up Java version definitions.

Author: yschimke