bumped github workflow to ubuntu 24.04 after 20.04 deprecation

Author: BartMassey

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   # Check code build succeeds.
   build-book-code:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
@@ -24,7 +24,7 @@ jobs:
 
   # Check build succeeds for microbit docs.
   build-book-doc:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
@@ -38,7 +38,7 @@ jobs:
 
   # Build the book HTML itself and optionally publish it.
   build-book:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1