github/workflows: Update Ubuntu images for code size CI tasks.

This updates the Ubuntu OS image the code size CI tasks run on, from
20.04-LTS to 22.04-LTS.  This is needed because 20.04 didn't have
packages that are needed to build RISC-V 32 code.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

.github/workflows/code_size.yml

@@ -24,7 +24,7 @@ concurrency:
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v4
       with:

.github/workflows/code_size_comment.yml

@@ -11,7 +11,7 @@ concurrency:
 
 jobs:
   comment:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - name: 'Download artifact'
         id: download-artifact