Migrate .java-version contents to .jruby.release

I think it makes more sense to use .jruby.release as the only file
for version-specific details. This change may need to be tweaked
to support JRuby 10 releases prior to 10.0.3.0 (assuming it is
merged into that release), or it may not be important since we
won't be changing the jruby.sh in any released artifacts.

This version is a backport to 9.4.15.0.

Author: headius

bin/.java-version

@@ -1,2 +1,3 @@
 JRUBY_VERSION=9.4.15.0-SNAPSHOT
 JRUBY_MAIN=org.jruby.Main
+JRUBY_MINIMUM_JAVA_VERSION=8

bin/.jruby.release

@@ -555,16 +555,24 @@ if [ -f "$JAVA_HOME/release" ]; then
     esac
 fi
 
-# Default java_runtime_version to $java_version
-: "${java_runtime_version:=$java_version}"
+# Get main class and version from .jruby.release
 
 # shellcheck source=/dev/null
-if [ -f "$JRUBY_HOME/bin/.java-version" ] && . "$JRUBY_HOME/bin/.java-version" && [ "${JRUBY_MINIMUM_JAVA_VERSION-}" ]; then
+# shellcheck disable=2153  # Assigned in sourced file
+if [ -f "$jruby_release_file" ] && . "$jruby_release_file"; then
+    java_class=$JRUBY_MAIN
+    jruby_version=$JRUBY_VERSION
     minimum_java_version=$JRUBY_MINIMUM_JAVA_VERSION
 else
-    # Only 9.4.12.0 and earlier will have shipped without a .java-version file, so fall back on minimum of 8
+    # Only 9.4.14.0 and earlier will have shipped without a .jruby.release file, so fall back on 9.4 defaults
+    java_class=org.jruby.Main
+    jruby_version=unspecified
     minimum_java_version=8
 fi
+
+# Default java_runtime_version to $java_version
+: "${java_runtime_version:=$java_version}"
+
 add_log "Detected Java version: $java_version"
 add_log "Detected Java runtime version: $java_runtime_version"
 
@@ -645,18 +653,6 @@ JAVA_OPTS="$JAVA_OPTS_TEMP"
 
 CP_DELIMITER=":"
 
-# Get main class and version from .jruby.release
-
-# shellcheck source=/dev/null
-# shellcheck disable=2153  # Assigned in sourced file
-if [ -f "$jruby_release_file" ] && . "$jruby_release_file"; then
-    java_class=$JRUBY_MAIN
-    jruby_version=$JRUBY_VERSION
-else
-    java_class=org.jruby.main
-    jruby_version=unspecified
-fi
-
 # Find main jruby jar and add it to the classpath
 jruby_jar=
 for j in "$JRUBY_HOME"/lib/jruby.jar "$JRUBY_HOME"/lib/jruby-complete.jar; do