test: adding Java 7 check

Using the same method as googleapis/google-http-java-client#1847
to use surefire's jvm system property

.github/sync-repo-settings.yaml

@@ -8,6 +8,7 @@ branchProtectionRules:
     requiresCodeOwnerReviews: true
     requiresStrictStatusChecks: false
     requiredStatusCheckContexts:
+      - units (7)
       - units (8)
       - units (11)
       - windows

.github/workflows/ci-java7.yaml

@@ -0,0 +1,55 @@
+# Copyright 2022 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# Github action job to test core java library features on
+# downstream client libraries before they are released.
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+name: ci-java7
+jobs:
+  units:
+    name: "units (7)"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: actions/setup-java@v1
+      # setup-java v2 or higher does not have version 1.7
+      with:
+        version: 1.7
+        architecture: x64
+    - run: |
+        java -version
+        # This value is used in "-Djvm=" later
+        echo "JAVA7_HOME=${JAVA_HOME}" >> $GITHUB_ENV
+    - uses: actions/setup-java@v3
+      with:
+        java-version: 17
+        distribution: temurin
+    - name: Set up Maven
+      uses: stCarolas/setup-maven@v4.5
+      with:
+        maven-version: 3.8.8
+    - name: Build
+      shell: bash
+      run: |
+        # Leveraging surefire's jvm option, running the test on Java 7.
+        # Surefire plugin 2.22.2 is the last version for Java 7. Newer version would fail with 
+        # "UnsupportedClassVersionError: org/apache/maven/surefire/booter/ForkedBooter" error.
+        mvn --batch-mode --show-version -ntp test \
+            -Dclirr.skip=true -Denforcer.skip=true -Dmaven.javadoc.skip=true \
+            -Dgcloud.download.skip=true -T 1C \
+            -Dproject.surefire.version=2.22.2 \
+            -Djvm=${JAVA7_HOME}/bin/java

pom.xml

@@ -293,7 +293,7 @@
         </plugin>
         <plugin>
           <artifactId>maven-surefire-plugin</artifactId>
-          <version>3.0.0-M7</version>
+          <version>${project.surefire.version}</version>
           <configuration>
             <argLine>-Xmx1024m</argLine>
             <reportNameSuffix>sponge_log</reportNameSuffix>
@@ -514,6 +514,8 @@
     <project.datanucleus-api-jdo.version>3.2.1</project.datanucleus-api-jdo.version>
     <project.datanucleus-maven-plugin.version>4.0.3</project.datanucleus-maven-plugin.version>
     <project.servlet-api.version>2.5</project.servlet-api.version>
+    <!-- Declaring surefire version as property lets us ot specify its version as Maven's argument -->
+    <project.surefire.version>3.0.0-M7</project.surefire.version>
     <deploy.autorelease>false</deploy.autorelease>
   </properties>