Cross-compile to Scala 2.13

[lolgab]

Summary

• Add Scala 2.13.16 to crossScalaVersions alongside 2.12.20, so sbt +compile/+publish build both.
• apollo-runtime dep is a plain Java artifact (%, not %%), so no cross-version lookup issue.

Test plan

• sbt +compile succeeds for both 2.12.20 and 2.13.16 locally.
• CI already runs sbt +compile / sbt +publish (cross build), no workflow changes needed.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

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[codacy-production]

Pull Request Overview

This PR successfully introduces support for Scala 2.13.16 alongside the existing 2.12.20 version. Automated analysis from Codacy shows that the project remains up to standards with no new quality issues or complexity increases introduced by these configuration changes.

The primary focus for this PR should be verifying that the cross-compilation actually succeeds in the CI environment. While the build.sbt changes enable the functionality, confirmation of successful artifact generation for both versions is required to satisfy the acceptance criteria.

Test suggestions

• Successful compilation under Scala 2.12.20
• Successful compilation under Scala 2.13.16

Prompt proposal for missing tests

Consider implementing these tests if applicable:
1. Successful compilation under Scala 2.12.20
2. Successful compilation under Scala 2.13.16

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