Clarify that `java-version=dev` requires `distribution=graalvm-community`

[rudsberg]

Using distribution=graalvm and java-version=dev fails:

  with:
    java-version: dev
    distribution: graalvm
    [...]
Error: Could not find highest Java version. Please file an issue at: https://github.com/graalvm/setup-graalvm/issues.

Expected:

• Clear validation/error statingjava-version=dev is unsupported for distribution=graalvm, and to instead use distribution=graalvm-community.

Actual:

• Generic “Could not find highest Java version” error.

Proposal:

• Note in docs that dev requires graalvm-community. For example:

- dev builds: 'dev'
+ dev builds: 'dev' (requires 'graalvm-community')

• Validation: Reject dev + graalvm with an actionable message.