mark support for installing extensions in parallel as being mature, since it's no longer experimental

[boegel]

Requires:

• don't raise error when required extensions are not found when installing extensions in parallel #4671
• add missing return for customised install_extension_async methods in Rserve and Rmpi easyblocks easybuild-easyblocks#3474
• use *DESCRIPTION rather than */DESCRIPTION in RPackage.requires_deps easybuild-easyblocks#3490