[bugfix] macOS .app: accept Java 21+ for JVM discovery

The appbundler-maven-plugin writes <jvmRequired> verbatim into the
.app bundle's Info.plist as JVMVersion. Apple's JavaAppLauncher uses
that key (and the convention "<major>+") to gate which installed JVM
it will use to launch the app. With <jvmRequired>21</jvmRequired> the
launcher's plist contains JVMVersion = 21 (a strict match), so a
host with only Java 22 / 23 / 24 / … installed would be rejected even
though eXist runs fine on those.

Switch to "21+" — matches the established Apple convention (1.6+,
1.7+, 1.8+, …) and lines up with how Zulu / Temurin / Liberica / Apple
JDKs all install their major-version directories. Verified the plugin
propagates the literal:

    /usr/libexec/PlistBuddy -c "Print :JVMVersion" Contents/Info.plist
    # 21+

Follow-up to @duncdrum review on PR #6434 (#6434 (comment)).

Author: joewiz

exist-distribution/pom.xml

@@ -923,7 +923,7 @@
                             <identifier>org.exist.start.Main</identifier>
                             <mainClassName>org.exist.start.Main</mainClassName>
                             <executableName>eXist-JavaAppLauncher</executableName>
-                            <jvmRequired>21</jvmRequired>
+                            <jvmRequired>21+</jvmRequired>
                             <minimumSystemVersion>10.9</minimumSystemVersion>
                             <version>${project.version}</version>
                             <shortVersion>${project.version}</shortVersion>