scripts/release.sh: ensure tree is clean before releasing (#3175)

Motivation:

As part of the release process our script makes some changes to versions
etc and then everything in the branch is committed. This is dangerous if
there are uncommitted changes present.

Modifications:

As part of the release.sh script check if the tree has uncommitted changes
and error out if it does.

Author: bryce-anderson

scripts/release.sh

@@ -53,6 +53,11 @@ if [ "$#" -ne "2" ]; then
     exit 1
 fi
 
+if [ -n "$(git status --porcelain)" ]; then
+  echo "Branch has uncommitted changes. Commit or revert any changes before releasing."
+  exit 1
+fi
+
 # Enforce JDK17 to get latest LTS javadoc format/features (search, etc.):
 java_version=$(./gradlew --no-daemon -version | grep 'Launcher JVM:' | \
    awk -F\. '{gsub(/Launcher JVM:[ \t]*/,"",$1); print $1"."$2}')