fix: update CI to use maven 3.8.1 (#662)

Gitbub Action updated LTS(18.04, 20.04) versions of Github-Hosted runners for Ubuntu with the latest version of Apache Maven. The new version of Ubuntu has Apache Maven 3.8.2 which is incompatible with flatten-maven-plugin:1.2.7. This caused all github actions using the latest Ubuntu version to fail with this error.

The immediate solution is to use this community action to install maven 3.8.1 at runtime.

.github/workflows/ci.yaml

@@ -11,6 +11,9 @@ jobs:
       matrix:
         java: [8, 11]
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with:
@@ -22,6 +25,9 @@ jobs:
   windows:
     runs-on: windows-latest
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with:
@@ -36,6 +42,9 @@ jobs:
       matrix:
         java: [8, 11]
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with:
@@ -45,6 +54,9 @@ jobs:
   linkage-monitor:
     runs-on: ubuntu-latest
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with:
@@ -58,6 +70,9 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with:
@@ -69,6 +84,9 @@ jobs:
   clirr:
     runs-on: ubuntu-latest
     steps:
+    - uses: stCarolas/setup-maven@v4
+      with:
+        maven-version: 3.8.1
     - uses: actions/checkout@v2
     - uses: actions/setup-java@v1
       with: