Replace obsolete libraries

Both the okhttp and the retrofit versions used in this library are obsolete. There are vulnerabilities as well. Updating retrofit would bring in the kotlin stdlib, further growing the already large dependency tree, and in my opinion retrofit's API classes are pretty annoying to work with considering the "Call" wrapping every result.

So my proposal is to replace both of these libraries with a new one. This means that Java 8 will be the minimum supported version. By doing this, the API clients will be easier to use and we will get rid of the vulnerability warnings faced by each and every user of appkit with an IDE.

Start by locating the okhttp and retrofit dependency declarations and the API clients that use them. Review the Java 8 compatibility requirements and evaluate the proposed replacement's impact on the public API. Done means both obsolete libraries are removed, vulnerability warnings no longer appear, and the clients remain usable with Java 8.

Written by the indexing model from the issue text.