[ci] can't use ubuntu 18.04 today

.github/workflows/main.yml

@@ -243,7 +243,7 @@ jobs:
 
 
   linux-build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       PLATFORM: linux64
       OPAMYES: 1
@@ -342,7 +342,7 @@ jobs:
 
   linux-test:
     needs: linux-build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       PLATFORM: linux64
       TEST: ${{matrix.target}}
@@ -424,7 +424,7 @@ jobs:
 
   test-docgen:
     needs: linux-build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       PLATFORM: linux64
       HXCPP_COMPILE_CACHE: ~/hxcache
@@ -500,7 +500,7 @@ jobs:
         working-directory: ${{github.workspace}}/tests/docgen
 
   linux-arm64:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     permissions:
       packages: write
     env:
@@ -921,7 +921,7 @@ jobs:
   deploy:
     if: success() && github.repository_owner == 'HaxeFoundation' && github.event_name != 'pull_request'
     needs: [linux-test, linux-arm64, mac-test, windows-test, windows64-test]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       # this is only needed for to get `COMMIT_DATE`...
       # maybe https://github.community/t/expose-commit-timestamp-in-the-github-context-data/16460/3
@@ -997,7 +997,7 @@ jobs:
   deploy_apidoc:
     if: success() && github.repository_owner == 'HaxeFoundation' && github.event_name != 'pull_request'
     needs: [linux-test, linux-arm64, mac-test, windows-test, windows64-test]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install dependencies
         run: |