Rename various qt6_add_repc_X functions

Context
https://git.qt.io/alcroito/qt6-cmake-api-review/-/commit/5313c017d09c51d9034dd62b33f968b9cd54363c#179fb5213a114272e340c63285f9fb35ba530ab8_265_303

The remote objects cmake api was released as public API in 6.1, so we can't just rename the functions.

Keep the old names for compatibility (qt6_add_repc_source, qt6_add_repc_replica), but add a warning to the implementation of those functions to use the new names instead (effectively the old names are deprecated).

qt6_add_repc_files should have been internal, but unfortunately it's already public API, so add a warning not to use it, and the warning should instead say to use of the
`qt6_add_repc_sources`, `qt6_add_repc_replicas`, `qt6_add_repc_merged` functions instead (this can be done by checking the type 'argument' passed into the function)

The function is now effectively deprecated.

Ensure to remove all the old names from docs.