Revert "[#77] Update to java 17"

This reverts commit cb3bb7e.

# Conflicts:
#	.github/workflows/build_girders.yml

girders-demos/pom.xml

@@ -23,9 +23,9 @@
   </modules>
 
   <properties>
-    <jdk.version>21</jdk.version>
-    <maven.compiler.source>21</maven.compiler.source>
-    <maven.compiler.target>21</maven.compiler.target>
+    <jdk.version>17</jdk.version>
+    <maven.compiler.source>17</maven.compiler.source>
+    <maven.compiler.target>17</maven.compiler.target>
     <!-- version properties -->
     <bootstrap.version>4.6.2</bootstrap.version>
     <girders.version>6.2.3-SNAPSHOT</girders.version>

girders-parent/pom.xml

@@ -16,8 +16,8 @@
   <description>Girders Parent</description>
 
   <properties>
-    <java.source.version>21</java.source.version>
-    <java.target.version>21</java.target.version>
+    <java.source.version>17</java.source.version>
+    <java.target.version>17</java.target.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <!-- plugin versions -->
     <checkstyle.version>10.12.1</checkstyle.version>

pom.xml

@@ -97,7 +97,7 @@
   </modules>
 
   <properties>
-    <jdk.version>21</jdk.version>
+    <jdk.version>17</jdk.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <coverage.minimum>0.8</coverage.minimum>

src/site/markdown/quickstart/index.md

@@ -38,7 +38,7 @@ pom.xml:
       
       <properties> 
         <!-- Set the major version number of Java that you want to use -->
-        <jdk.version>21</jdk.version>
+        <jdk.version>17</jdk.version>
         
         <!-- This is the version of Girders you want to use. Ideally, this is the only version number that you have
           to configure for the dependency management. -->