MINOR: [CI] Ignore Checkstyle 13+ in Dependabot (requires Java 21)

[jbonofre]

Rationale for this change

Dependabot PR #1324 bumps Checkstyle from 10.23.0 to 14.3.0 and fails on every JDK 17 job with:

UnsupportedClassVersionError: com/puppycrawl/tools/checkstyle/api/CheckstyleException has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 61.0

Checkstyle 13.0.0 and later are built for Java 21, while the build still runs on JDK 17. The 12.x line is the last one built for Java 17.

What changes are included in this PR?

Add an ignore rule to the Maven ecosystem in .github/dependabot.yml so Dependabot no longer proposes Checkstyle >=13. Updates within 12.x are still proposed.

Are these changes tested?

No, this is a CI configuration change only. The YAML was checked to parse.

Are there any user-facing changes?

No.

[kou]

We may not need this because you already used @dependabot ignore this major version in #1324 .

[jbonofre]

@kou yes, you are right. I created this PR before closing #1324. Let me close this one.