Bump Scala to 2.12.20

[Fokko]

Adds support for JDK21+:

• https://github.com/scala/scala/releases/tag/v2.12.18
• https://github.com/scala/scala/releases/tag/v2.12.19
• https://github.com/scala/scala/releases/tag/v2.12.20

Or see: https://endoflife.date/scala#jdk-compatibility

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[pan3793]

Is the parquet-scala module actually being used?

[Fokko]

@pan3793 I don't have full visibility on that, but it doesn't like it, looking at Usages in Maven Central: https://mvnrepository.com/artifact/org.apache.parquet/parquet-scala

Might be good to see if we can retire some modules that are not used anymore (I think parquet-pig is also another good candidate).

[pan3793]

+1 for retiring the parquet-scala module, or even dropping it directly.

there is only one class in the parquet-scala module, users are easy to copy and embed it if we drop the parquet-scala module.

additionally, it's rare to publish a scala lib that only supports a single scala binary version, most scala libs support 2~3 scala binary versions in each version

[Fokko]

@pan3793 I agree, are you interested in raising a [DISCUSS] on the mailinglist?

[pan3793]

@Fokko https://lists.apache.org/thread/scdq9t2gvvs4glhq0qx4qcvfp62j793s