Release a new version of gson-extras

I have been operating under the assumption that the [`extras`](https://github.com/google/gson/tree/main/extras) project had no Maven Central artifacts, but that's not true! We have [`com.google.code.gson:gson-extras`](https://mvnrepository.com/artifact/com.google.code.gson/gson-extras/2.8.5), last released in 2018. Not much has happened since, but there is at least a change to `RuntimeTypeAdapterFactory` to give it a [`recognizeSubtypes()` method](https://github.com/google/gson/blob/528fd3195bad9c6c816e77c96750b3188a514365/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java#L207). The existing artifact also has a CVE logged against it, and even though that is actually not a security problem at all, it would be nicer for it not to be there.

I tried releasing a new version with the existing configuration, but it didn't work. It got as far as making a git tag and everything, but then said:
```
[INFO] [INFO] --- maven-deploy-plugin:3.1.3:deploy (default-deploy) @ gson-extras ---
[INFO] [INFO] Skipping artifact deployment
```

I'm sure there's some refinement needed to `extras/pom.xml`, probably similar to what's in `gson/pom.xml`.

We should also correct [this statement](https://github.com/google/gson/blob/528fd3195bad9c6c816e77c96750b3188a514365/extras/README.md?plain=1#L6) in the README:
> The artifacts created by this module are currently not deployed to Maven Central.

(It's technically correct, I suppose, since it says that "are currently not deployed", not that they never have been deployed.)