TCK allow certification against Java 17 and 21

[KyleAure]

Based on jakartaee/platform#820
The TCK must be able to certify against Java 17 and 21.
Thankfully, we wrote the TCK using Java 17 and never used any features from Java 21.
To formalize this we need to:

• Update TCK documentation
• Remove TODOs for Java 21
• Update certification issue template to allow choice of Java level