Bump to llvm version 18 in CI

[mattisboeckle]

We are running into problems where the versions used for development and for the CI are differing so much that we sometimes write code in development that is invalid in the CI. (See #877)

I propose upping the Version to 18, which seems to be the Ubuntu standard

Edit: the latest release is 20.1.0, so we are also not in unstable territory

[jiribenes]

Note that this needs to be coordinated as it also requires changes at least:

1. on the website

2. in the autorelease CI,
   

   effekt/.github/workflows/autorelease.yml

   Lines 124 to 125 in 19fc826

   	- name: Install Chez Scheme, LLVM & libuv
   	run: sudo apt-get install -y chezscheme llvm-15 libuv1-dev

3. in the config,
   

   effekt/effekt/jvm/src/main/scala/effekt/EffektConfig.scala

   Line 72 in 19fc826

   default = Some(sys.env.getOrElse("EFFEKT_LLVM_VERSION", "15")),

4. and in the runner!
   

   effekt/effekt/jvm/src/main/scala/effekt/Runner.scala

   Line 270 in 19fc826

   lazy val llcCmd = discoverExecutable(List("llc", "llc-15", "llc-16"), List("--version"))

plus in the packaging in effekt-nix (but I can take care of that part) for the community build(s)

[marvinborner]

Are we sure we want to make version 18 a requirement? The default version in current Debian (which is probably which most people install) is still below 18 (or even 15, see here).

Either way, we'd also need to sync this change with the plots ci (which uses the default Ubuntu version)

[jiribenes]

FWIW, I'm fine with pushing Debian users into using the official apt packages via https://apt.llvm.org/

[mattisboeckle]

Changed the remaining locations I could find. I think we should rebase this on top of #888.

[jiribenes]

We still need to update the website in sync as it mentions the version requirements: https://github.com/effekt-lang/effekt-website/blob/d2f0fee403ef48777465f48f0998a42cd35ed0e9/contributing.md?plain=1#L34

[mattisboeckle]

effekt-website also has a PR now.

effekt-lang/effekt-website#84

[jiribenes]

LGTM