Bump Java to 21

This is to satisfy a dependency requirement pending in a package update.
PR 145 bumps Jena to version 6.0, and Jena's release page notes:

> Jena6 requires Java 21, or a later version of Java.

References:
* #145
* https://jena.apache.org/download/

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

.github/workflows/build.yml

@@ -88,7 +88,7 @@ jobs:
         uses: actions/setup-java@v4.2.2
         with:
             distribution: 'temurin'
-            java-version: '18'
+            java-version: '21'
             cache: 'maven'
 
       # Build the project