Migrate projects from JDK 17 to JDK 21

Our projects are currently compiled with version 17 of the JDK. To better align with current platforms, we should upgrade to the latest Java LTS, which is currently version 21. The SDK will be the first update as the base for other projects, but we will want to update all Java APIs and make sure that the following places have had updates to their references:

• README requirements
• Build image in Jenkinsfile
• Dockerfiles for APIs
• POM compiler targets

Additionally, we should consider looking into using Temurin JDKs as it is something shipped from an Eclipse project over the OpenJDK we currently use.