Remove runtime dependency on maven-artifact

[ryanrupp]

This was primarily being used to check the Java version to handle < Java 7 in a different way for date formatting, but since Java 8 is the minimum version at this point it is no longer necessary. Also ends up removing the transitive dependency on commons lang which had minimal usage but replaced with an internal StringUtils equivalent.

[ryanrupp]

Will note this change came in as part of this commit but the compiler target at that point was already 1.8 (so there should be no Java 6 runtime users unless someone is manually compiling this with a change target of 1.6). Unclear what the minimum Java version is supposed to be, I would personally say Java 8 is reasonable at this point, looks like at some point this was actually at Java 11 but pulled back to 8 here.

[ryanrupp]

I also confirmed the latest release (6.2.2) on maven central is indeed producing Java 8 output bytecode (in case there was another build system/config doing the actual publishing).

file com/messagebird/APIResponse.class
com/messagebird/APIResponse.class: compiled Java class data, version 52.0 (Java 1.8)

[ryanrupp]

I guess would maybe use this PR (or a follow up) to call out the minimum Java version requirement in the README