ci: use newer actions/x versions for master jobs

Now that the jobs are running again, a number of deprecation warnings
have surfaced
https://github.com/libevent/libevent/actions/runs/4876367881:
```bash
Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16: actions/checkout@v2.0.0.
```

Looks like the most straightforward thing todo here is just use the
newer version of the checkout action.

Also move to the v1 tag for the coveralls job, rather than a pinned,
much older version (the latest release is 1.2.5, https://github.com/coverallsapp/github-action/releases).

Same again for actions/cache.

.github/workflows/master.yml

@@ -15,9 +15,9 @@ jobs:
       contents: read  # for actions/checkout to fetch code
     runs-on: ubuntu-20.04
     steps:
-      - uses: actions/checkout@v2.0.0
+      - uses: actions/checkout@v3
       - name: Cache
-        uses: actions/cache@v1.0.3
+        uses: actions/cache@v3
         with:
           path: build
           key: ${{ matrix.os }}-coverage-v2
@@ -43,7 +43,7 @@ jobs:
             make verify_coverage
 
       - name: Coveralls GitHub Action
-        uses: coverallsapp/github-action@v1.0.1
+        uses: coverallsapp/github-action@v1
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           path-to-lcov: ./build/coverage.info.cleaned
@@ -65,7 +65,7 @@ jobs:
       fail-fast: false
 
     steps:
-      - uses: actions/checkout@v2.0.0
+      - uses: actions/checkout@v3
 
       - name: Install Dependencies
         run:
@@ -126,7 +126,7 @@ jobs:
       fail-fast: false
 
     steps:
-      - uses: actions/checkout@v2.0.0
+      - uses: actions/checkout@v3
       - name: Install Depends
         run: |
           sudo apt install doxygen libmbedtls-dev