[develop page] Incorrect semver for pre-releases

[SpaceWalkerRS]

The semver for pre-releases should always use rc as the pre-release identifier (e.g. 1.8-rc.1). FLoader uses beta instead for mc versions after 1.16 but that is not relevant for us.

At the moment the dev page gives results like 1.8-pre.1. It seems the pre-release pattern matching is not working at all.

[thdaele]

These 2 PR's have landed since we introduced minecraft_semver.js, I assume fixing this bug should be done by mirroring what these PR's have done.
FabricMC/fabric-loader#1041
FabricMC/fabric-loader#1072