workflows: ubuntu 18.04 is deprecated

And will soon be removed.

.github/workflows/build.yml

@@ -77,7 +77,7 @@ jobs:
       CCACHE_TEMPDIR: /tmp/.ccache-temp
     strategy:
       matrix:
-        os: [ubuntu-latest, ubuntu-18.04]
+        os: [ubuntu-22.04, ubuntu-20.04]
     steps:
     - uses: actions/checkout@v1
       with:
@@ -101,7 +101,7 @@ jobs:
         ${{env.BUILD_DEFAULT_LINUX}}
 
   libwallet-ubuntu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CCACHE_TEMPDIR: /tmp/.ccache-temp
     steps:
@@ -129,7 +129,7 @@ jobs:
 
   test-ubuntu:
     needs: build-ubuntu
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CCACHE_TEMPDIR: /tmp/.ccache-temp
     steps:

.github/workflows/depends.yml

@@ -18,7 +18,7 @@ env:
 
 jobs:
   build-cross:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CCACHE_TEMPDIR: /tmp/.ccache-temp
     strategy: