The general guidance is always use the last LTS release unless there is a specific feature in the non LTS releases that you can’t wait for. So any new development should target 17 at this point. Unless you want serious long term pain then you shouldn’t be more than one LTS release behind. So if you are not running on JDK 11 or later you should be strongly thinking about upgrading.
Why should I stick to LTS? All other versions aren't in any way less tested, you should always stick to the newest released JDK version, be it LTS or not, this way you get all the benefits (language features, performance gains) and security ones (security fixes always first land in the newest version, and are backported to the older ones). Upgrades now are pretty straightforward if you are past JKD 9 - with JDK 16-17…
That depends. If most people follow the advice of "sticking to LTS", these versions will have way more users, and thus will be more "battle-tested", as in tested in production.