Change runners to use Ubuntu 20.04

Seems like GitHub finally pulled the plug out of 18.04.

.github/workflows/build.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - { name: ubuntu-18.04,       os: ubuntu-18.04,   cross-target: '' }
+          - { name: ubuntu-20.04,       os: ubuntu-20.04,   cross-target: '' }
           - { name: macos-universal,    os: macos-11,       cross-target: aarch64-apple-darwin }
           - { name: windows,            os: windows-latest, cross-target: '' }
     name: Package plugin binaries

.github/workflows/docs.yml

@@ -13,7 +13,7 @@ defaults:
 jobs:
   docs:
     name: Generate and upload docs
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       # Needed for git-describe to do anything useful

.github/workflows/test.yml

@@ -15,7 +15,7 @@ jobs:
   test:
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-11, windows-latest]
+        os: [ubuntu-20.04, macos-11, windows-latest]
     name: Build and test all components
     runs-on: ${{ matrix.os }}
     steps: