Merge #6146

6146: Instruct to use rustup/rustc-mozilla on older Debian/Ubuntu (fixes #6145) r=Chairn a=def-

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## Checklist

- [ ] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Dennis Felsing <dennis@felsin9.de>

README.md

@@ -39,6 +39,8 @@ You can install the required libraries on your system, `touch CMakeLists.txt` an
 
 On older distributions like Ubuntu 18.04 don't install `google-mock`, but instead set `-DDOWNLOAD_GTEST=ON` when building to get a more recent gtest/gmock version.
 
+On older distributions `rustc` version might be too old, to get an up-to-date Rust compiler you can use [rustup](https://rustup.rs/) with stable channel instead or try the `rustc-mozilla` package.
+
 Or on CentOS, RedHat and AlmaLinux like this:
 
     sudo yum install cargo cmake ffmpeg-devel freetype-devel gcc gcc-c++ git glew-devel glslang gmock-devel gtest-devel libcurl-devel libnotify-devel libogg-devel libpng-devel libx264-devel make openssl-devel opus-devel opusfile-devel python2 rust SDL2-devel spirv-tools sqlite-devel vulkan-devel wavpack-devel