Update jknack handlebars to the latest version and reuse its cache

**Is your feature request related to a problem? Please describe.**
Currently, Knot.x Handlebars TE Strategy uses ` implementation(group = "com.github.jknack", name= "handlebars", version = "4.0.6")`.
There is a newer version available.

**Describe the solution you'd like**
Upgrade `handlebars` lib to the latest version and reuse its [cache](https://github.com/jknack/handlebars.java#the-cache-system) instead of existing Guava cache. It would be a good idea to make it configurable.

**Describe alternatives you've considered**
If it would be impossible to keep the cache config, consider leaving the current one.

Start with the Handlebars dependency declaration at version 4.0.6 and the linked Handlebars cache-system documentation. Confirm the target version and how cache configuration should remain compatible; done means the dependency is upgraded and the existing cache is replaced or retained as documented.

Written by the indexing model from the issue text.