Replace SpongeCommon.server().registryAccess() -> SpongeCommon.vanillaRegistryAccess()

[MrHell228]

vanillaRegistryAccess() essentially does the same, simply hiding static access to server for places that only need registry access. As discussed in discord, these calls to server may happen before server is actually created, meaning the content of vanillaRegistryAccess() is likely subject to change later. So this PR is just cleanup and preparation for possible future