[docs] Document JDK 21 since Logstash 8.15.0

[toby-sutor]

Since Logstash 8.15.0, we are shipping JDK 21 as per https://www.elastic.co/guide/en/logstash/current/logstash-8-15-0.html and #16055:

IMPLEMENTOR="Eclipse Adoptium"
IMPLEMENTOR_VERSION="Temurin-21.0.5+11"
JAVA_RUNTIME_VERSION="21.0.5+11-LTS"
JAVA_VERSION="21.0.5"
JAVA_VERSION_DATE="2024-10-15"
LIBC="gnu"
MODULES="java.base java.compiler java.datatransfer java.xml java.prefs java.desktop java.instrument java.logging java.management java.security.sasl java.naming java.rmi java.management.rmi java.net.http java.scripting java.security.jgss java.transaction.xa java.sql java.sql.rowset java.xml.crypto java.se java.smartcardio jdk.accessibility jdk.internal.jvmstat jdk.attach jdk.charsets jdk.internal.opt jdk.zipfs jdk.compiler jdk.crypto.ec jdk.crypto.cryptoki jdk.dynalink jdk.internal.ed jdk.editpad jdk.hotspot.agent jdk.httpserver jdk.incubator.vector jdk.internal.le jdk.internal.vm.ci jdk.internal.vm.compiler jdk.internal.vm.compiler.management jdk.jartool jdk.javadoc jdk.jcmd jdk.management jdk.management.agent jdk.jconsole jdk.jdeps jdk.jdwp.agent jdk.jdi jdk.jfr jdk.jlink jdk.jpackage jdk.jshell jdk.jsobject jdk.jstatd jdk.localedata jdk.management.jfr jdk.naming.dns jdk.naming.rmi jdk.net jdk.nio.mapmode jdk.random jdk.sctp jdk.security.auth jdk.security.jgss jdk.unsupported jdk.unsupported.desktop jdk.xml.dom"
OS_ARCH="x86_64"
OS_NAME="Linux"
SOURCE=".:git:9f7db4edca1b"
BUILD_SOURCE="git:08621c72262ba260e4bb6451a9c75bf1c0ab365d"
BUILD_SOURCE_REPO="https://github.com/adoptium/temurin-build.git"
SOURCE_REPO="https://github.com/adoptium/jdk21u.git"
FULL_VERSION="21.0.5+11-LTS"
SEMANTIC_VERSION="21.0.5+11"
BUILD_INFO="OS: Linux Version: 6.5.0-1025-azure"
JVM_VARIANT="Hotspot"
JVM_VERSION="21.0.5+11-LTS"
IMAGE_TYPE="JDK"

This needs to be reflected in the documentation and backported to 8.15.0 and newer. Ideally, it would be reviewed by the dev team to ensure all other information is still accurate too.

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_16780.docs-preview.app.elstc.co/diff

[mergify]

This pull request does not have a backport label. Could you fix it @toby-sutor? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[mergify]

This pull request is now in conflicts. Could you fix it @toby-sutor? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b toby-sutor-patch-1 upstream/toby-sutor-patch-1
git merge upstream/main
git push upstream toby-sutor-patch-1