Actions: remove Ubuntu 18.04 runner

The Ubuntu 18.04 runner is deprecated and will be fully removed on
2022-12-01, after which the jobs will fail.

Let's remove the Ubuntu 18.04 runner.

Reference: actions/runner-images#6002

.github/workflows/integration.yml

@@ -15,7 +15,7 @@ jobs:
     name: ${{ matrix.os }} JDK 8
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-22.04]
+        os: [ubuntu-20.04, ubuntu-22.04]
     runs-on: ${{ matrix.os }}
     env:
       NODE_VERSION: "lts/*"

docs/dg/devOpsGuide.md

@@ -31,7 +31,7 @@ All three types of tests in the test suite are run in a single GitHub Actions wo
 1. Ubuntu JDK 8 (`ubuntu`): Runs both unit tests and system tests on JDK 1.8 running on supported Ubuntu versions. This job also produces a RepoSense report and the MarkBind documentation website for previewing.
 2. macOS JDK 8 (`macos`): Runs both unit tests and system tests on JDK 1.8 running on supported macOS versions.
 3. Windows JDK 8 (`windows`): Runs both unit tests and system tests on JDK 1.8 running on supported Windows versions.
-4. Cypress frontend tests (`cypress`): Runs only the frontend tests on JDK 1.8 running on Ubuntu 18.04 LTS.
+4. Cypress frontend tests (`cypress`): Runs only the frontend tests on JDK 1.8 running on Ubuntu 20.04 LTS.
 
 The list of supported OS versions are [available on the GitHub Docs website](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources). These jobs should be updated regularly whenever RepoSense supports newer versions of the Java Development Kit (JDK) as well as when new OS versions are made available (via the job OS matrix). Frontend tests are run separately to take advantage of parallel job execution.