Drop build support for Java 1.7 and 1.8

Keep producing 1.7 artifacts, but use a more modern compiler to do
so.

We used to rely on source/target flags to support an earlier
version of Java, but had to run 1.7 in CI to verify we didn't
accidentally use newer APIs.

While Java 1.7 is available in GH actions, maven doesn't quite
work. With Java 9 we can use release flag instead of source/target,
which also checks that we only use classes compatible with 1.7
classpath, so we don't have to run maven 1.7 in CI.

Author: vickenty

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        java-version: ['7', '8', '11', '13', '17']
+        java-version: ['9', '11', '13', '17']
         include:
           - os: windows-latest
             java-version: '12'

.gitlab-ci.yml

@@ -17,7 +17,7 @@ deploy_to_sonatype:
   tags:
     - "docker-in-docker:amd64"
 
-  image: maven:3.9-eclipse-temurin-8
+  image: maven:3.9-eclipse-temurin-17
 
   script:
     # Ensure we don't print commands being run to the logs during credential

pom.xml

@@ -347,7 +347,7 @@
                         <configuration>
                             <rules>
                                 <requireJavaVersion>
-                                    <version>[1.7.0-0,1.9.0-0),[9.0,18.0-0)</version>
+                                    <version>[9.0,18.0-0)</version>
                                 </requireJavaVersion>
                             </rules>
                         </configuration>
@@ -372,8 +372,7 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.8.1</version>
                 <configuration>
-                    <source>1.7</source>
-                    <target>1.7</target>
+                    <release>7</release>
                 </configuration>
             </plugin>
             <plugin>