Add 2.13.13 and 2.12.19 to supported bridges list

[lolgab]

The bridges are already on Maven Central.

Pull Request: #3066

[lefou]

They aren't needed anywhere except when trying to publish them with the all value in MILL_COMPILER_BRIDGE_VERSIONS env var, right?

It would be good, if we could set this env var in some CI job, to cover any issues with bridge modules.

[lolgab]

No, they are used by mill to decide if try to get the precompiled bridge via coursier, or to compile the bridge in place. With current mill if you use Scala 2.13.13 you see the Compiling compiler bridge log.
Ideally IMO, we would try anyways, and then in case we get a not found from coursier mill should fallback to compile the bridge in place. Otherwise you lose the precompiled bridge if you don't update Mill to the latest snapshot when a new version of Scala is released.

[lefou]

That means, no older Mill version is using the bridges. I also think we should always try to download them.