Auto-check Release Phase for Breaking Changes

# What
Have a CI check or whatever that prevents maintainers from merging PR that are labeled w/ https://github.com/apache/arrow-rs/labels/api-change or https://github.com/apache/arrow-rs/labels/next-major-release while we're on a "minor release" phase.

# Why
[The contribution guideline say](https://github.com/apache/arrow-rs/blob/main/CONTRIBUTING.md#breaking-changes) that breaking changes should only be merged according to the [release schedule](https://github.com/apache/arrow-rs/blob/main/README.md#release-versioning-and-schedule). As a maintainer with a reasonable busy day, I don't have a the mental capacity to manually check that for every PR.

See #7229 for example.

# How
I dunno, use a CI script or something. That would obviously better if we would have a proper merge queue.

Read the breaking-changes section of CONTRIBUTING.md and the release versioning and schedule section of README.md first, then review issue #7229. Define how the CI check should determine the current release phase and handle PRs carrying the api-change or next-major-release labels; done means those merges are prevented during a minor-release phase.

Written by the indexing model from the issue text.