[build] Move Spark Scala 2.13 tests to nightly

[litiliu]

Purpose

Linked issue: close #3936

Move the Spark Scala 2.13 compatibility lane out of the default pull-request CI to reduce duplicated load on the self-hosted runners, while preserving the existing coverage in Nightly.

Brief change log

• Exclude spark3-scala213 from reusable CI workflow calls by default.
• Enable spark3-scala213 explicitly in the Nightly workflow.
• Allow the Nightly workflow to be triggered manually when Scala 2.13 coverage is needed.
• Preserve the existing Scala 2.13 Spark module and test selection from stage.sh.

Tests

• actionlint .github/workflows/ci-template.yaml .github/workflows/ci.yaml .github/workflows/nightly.yaml
• bash -n .github/workflows/stage.sh
• Verified ./.github/workflows/stage.sh spark3-scala213 still selects -Pscala-2.13, excludes SparkLakeTest, and runs the existing Spark modules.
• ./mvnw spotless:check
• git diff --check apache/main...HEAD

API and Format

No public API or storage format changes.

Documentation

No documentation changes. This change only updates CI workflow scheduling.

Generative AI disclosure:

• No generative AI tools used
• Yes: Codex (OpenAI)

Generated-by: Codex (OpenAI) following the guidelines