docs: update stale discord invite links (#18588)

Author: mtnash

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 If you are a developer who wishes to contribute to the development of _RetroArch_; or if you have
 found a bug and wish to submit a minor patch and/or bug report, please read this document.
 
-Active discussions happen on our [Discord](https://discordapp.com/invite/27Xxm2h), mostly within
+Active discussions happen on our [Discord](https://discord.gg/C4amCeV), mostly within
 the _Programming_ channel category. We value discussions that happen in real time around
 these contributions.
 
@@ -64,7 +64,7 @@ may be merged in when it is fully approved. The final approval of merge requests
 discretion of the project.
 
 If you want to develop a larger feature or make broad changes, please do join our
-[Discord](https://discordapp.com/invite/27Xxm2h) server to discuss. The discussion is
+[Discord](https://discord.gg/C4amCeV) server to discuss. The discussion is
 necessary to prevent the possibility of major work being done which will not be accepted at all.
 
 ## libretro API

README.md

@@ -34,7 +34,7 @@ Latest binaries are currently hosted on the [buildbot](http://buildbot.libretro.
 
 ## Support
 
-To reach developers, either make an issue here on GitHub, make a thread on the [forum](https://www.libretro.com/forums/), chat on [discord](https://discord.gg/C4amCeV), or visit our IRC channel: #retroarch @ irc.freenode.org. You could create a post in [Reddit](https://www.reddit.com/r/RetroArch/) with *Technical Support* flair.
+To reach developers, either make an issue here on GitHub, make a thread on the [forum](https://www.libretro.com/forums/), chat on [Discord](https://discord.gg/C4amCeV), or visit our IRC channel: #retroarch @ irc.freenode.org. You could create a post in [Reddit](https://www.reddit.com/r/RetroArch/) with *Technical Support* flair.
 
 ## Documentation
 
@@ -339,7 +339,7 @@ The links below belong to our official channels. Links other than this may have
 - [YouTube Topic](https://www.youtube.com/channel/UC5q007PYyQPgin0HHbzF0zQ)
 - [Patreon](https://www.patreon.com/libretro)
 - [BOUNTYSOURCE](https://www.bountysource.com/teams/libretro/issues)
-- [Discord](https://discord.com/invite/VZ2b7wghxR)
+- [Discord](https://discord.gg/C4amCeV)
 - [Teespring](https://teespring.com/stores/retroarch)
 - [Documentation](https://docs.libretro.com/)
 - [Forum](https://forums.libretro.com/)