Suppress pkl-gradle POM metadata warning

[RushikeshGandhmal]

Fixes #1743

Summary

The pkl-gradle plugin intentionally requests the shadowed pkl-tools runtime variant. Gradle module metadata preserves that bundling attribute, but Maven POM metadata cannot represent it fully, so Gradle reports a compatibility warning when generating the pluginMaven POM.

Suppress the warning only for the runtimeElements variant of pluginMaven. Other publications and variants are unaffected.

Verification

• Reproduced the warning from a clean, current upstream/main checkout.
• Repeated the same POM-generation command after the change and confirmed the warning is absent.
• Ran :pkl-gradle:validatePom, :pkl-gradle:spotlessCheck, and :pkl-gradle:build successfully.
• Compared the generated POM and Gradle module metadata with the baseline; both files are byte-for-byte identical.
• Published the plugin and plugin marker to an isolated Maven-local repository using a temporary test signing key, then verified the POM, module metadata, JARs, marker, and signatures.
• Ran the Sonatype publication path with --dry-run; no artifacts were uploaded.
• Ran git diff --check.

[RushikeshGandhmal]

Hi @bioball, please find my testing below,

I verified the reported behavior manually using the same POM-generation command before and after the change.

On a clean, current upstream/main checkout, generating the pluginMaven POM succeeds but reports the runtimeElements metadata warning for the :pkl-tools dependency declared with Gradle attributes:

After applying this change, the same command succeeds without that warning:

The generated Maven POM and Gradle module metadata were also compared with the baseline and are byte-for-byte identical. The change only suppresses the known compatibility warning; it does not alter the published metadata.