GHA's Ubuntu 18 is deprecated

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

.github/workflows/ci.yml

@@ -4,12 +4,11 @@ on: [push, pull_request]
 
 jobs:
   sphinx:
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-22.04
 
     strategy:
       fail-fast: false
       matrix:
-        os: [ 'ubuntu-18.04' ]
         ruby: [ '2.7', '3.0', '3.1', '3.2' ]
         rails: [ '5_0', '5_1', '5_2', '6_0', '6_1', '7_0' ]
         database: [ 'mysql2', 'postgresql' ]
@@ -69,7 +68,7 @@ jobs:
         timeout-minutes: 12
 
   manticore:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     strategy:
       fail-fast: false