Change quarkus-cache extension status to stable

As discussed in a recent email conversation, `quarkus-cache` is currently marked as both `Supported` and `Preview` [here](https://code.quarkus.redhat.com/?extension-search=origin:platform%20quarkus-cache) which sends a mixed message. The extension is stable enough to be marked as `stable` which will remove the `Preview` tag.

cc @geoand @aloubyansky @cescoffier