ci: Build release on Ubuntu 18.04

Author: varungandhi-src

.github/workflows/release-template.md

@@ -9,3 +9,5 @@ chmod +x scip-clang
 ```
 
 The `-dev` binaries are meant for debugging issues (for example, if you run into a crash with `scip-clang`), and are not recommended for general use.
+
+Linux binaries should work with Ubuntu 18.04 (Debian Buster) or newer.

.github/workflows/release.yml

@@ -21,7 +21,7 @@ jobs:
       matrix:
         # NOTE: GitHub-hosted runners for macOS are x86_64 only
         # https://github.com/github/roadmap/issues/528
-        platform: ['ubuntu-20.04', 'macos-12'] # , 'windows-2022']
+        platform: ['ubuntu-18.04', 'macos-12'] # , 'windows-2022']
         config: ['dev', 'release']
         exclude:
           - platform: 'macos-12'