[FLINK-32734][build] Adds .mvn/maven.config to .gitignore

[XComp]

What is the purpose of the change

This allows the checkout-specific customization of the Maven build (e.g. setting a different local repository).

Brief change log

• Added .mvn/maven.config to .gitignore

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 2dbbb0c Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[snuyanzin]

LGTM

P.S. In theory support of user properties should help here
however it was added in 4.0.0 only https://issues.apache.org/jira/browse/MNG-6303

[XComp]

I'm merging this one despite the feature freeze because it's unrelated to the Flink code base.