[java-shared-config] fmt-maven-plugin versioning

It was raised in https://github.com/googleapis/java-storage-nio/issues/1069 that the fmt-maven-plugin has moved to com.spotify.fmt

It looks like the fmt-maven-plugin dependency is [pinned to specific version](https://github.com/googleapis/java-shared-config/pull/90) due to Java 8 compatibility. I believe that would mean we can't actually depend on com.spotify.fmt instead.

Is that the case, or would there be any way to move the dependency?

If we can't change the dependency feel free to close this, just wanted to make sure it's documented!

Start by reviewing the fmt-maven-plugin dependency pin in java-shared-config and the linked pull request 90, then check whether its version is constrained by Java 8 compatibility. Compare that constraint with the com.spotify.fmt coordinates referenced in the linked java-storage-nio issue. Done means documenting whether migration is possible or confirming that the issue can be closed.

Written by the indexing model from the issue text.