update CI config

Author: LordSimal

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ on:
 
 jobs:
   testsuite-linux:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
@@ -62,7 +62,7 @@ jobs:
 
   cs-stan:
     name: Coding Standard & Static Analysis
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
 
     steps:
     - uses: actions/checkout@v2