Create forward-looking policy to update language version requirements

There is no clear policy on which versions of Clojure/Script or Java Metosin Libraries are expected to support.

* Clojure
* 1.10 => 17.12.2018
* 1.11 => 22.3.2022

Java
* 11 => 11.9.2018

Good policy could be to support 12 (or 18 months) old non-latest stabile version, e.g. support for Clojure 1.10 could be dropped on 22.3.2023 when Clojure 1.11 turns 1 years.

Supporting old language versions is just extra work for little gain. If you want to use bleeding edge OS library, update language deps too. Clojure is good for keeping backwards compatability.

Start by reviewing the issue's proposed support windows for Clojure, ClojureScript, and Java. Define a forward-looking language-version policy for Metosin libraries, including when older versions may be dropped; the work is done when the policy is documented and its support timelines are clear.

Written by the indexing model from the issue text.