Update the version string in mod_proxy_cluster.h

[rhusar]

https://github.com/modcluster/mod_proxy_cluster/blob/main/native/include/mod_proxy_cluster.h#L21

Side question: can we include the git hash somehow?

[rhusar]

Note that the version is available via VERSION command - https://github.com/modcluster/mod_proxy_cluster/blob/main/native/mod_manager/mod_manager.c#L2384

So we need to make sure the name is correctly consumed, although, I don't think this is available / used anywhere (officially) yet.

[jajik]

related to #246

[jajik]

Once we release the beta, should we update the name as well (mod_cluster -> mod_proxy_cluster)? Because we discussed the versioning policy only...