Remove Gitter IRC bridge reference from documentation

[golanv]

The documentation states that the Gitter (Matrix) and IRC rooms are bridged via the Gitter IRC bridge. As best as I can tell, Gitter no longer maintains an IRC bridge. The project repository is archived, and the DNS records for irc.gitter.im appears to have been deleted.

Unless someone can host an IRC bridge (or find a public one), it may be better to remove this reference from the documentation.

[golanv]

I assume the IRC channel mentioned in the documentation is #spacemacs on libera.chat. There are about a dozen users in there, but the channel is unregistered and has no op. If the documentation should retain the reference to IRC, it may be a good idea to 1) mention #spacemacs@libera.chat specifically in the documentation (or the correct channel and network, if my assumption here is incorrect), and 2) the project owner may need to reach out to staff at libera.chat to regain ownership of the channel.