bump ci image to ubuntu-22.04, actions/checkout to v4 to avoid deprecations

Author: klensy

.github/workflows/ci.yml

@@ -5,9 +5,9 @@ name: continuous integration
 jobs:
   test:
     name: test
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
@@ -30,9 +30,9 @@ jobs:
 
   fmt:
     name: rustfmt
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal