Remove TestBench dependencies from the published POM #92

Consolidates the ChipFieldAddon items of FlowingCode/AddonsInternal#214 and FlowingCode/AddonsInternal#215, which are otherwise done across the other add-ons.

• #214: mark vaadin-testbench and testbench-rpc as <?dev-only?> and add dev-nuke-maven-plugin, so they are stripped from the published POM.
• #215: add a <?dev-only?> repository entry for https://maven.flowingcode.com/releases and bump testbench-rpc to 1.5.1, which is no longer published to Maven Central.

Both are implemented in #91.

Blocked

#91 fails the build-vaadin14 (JDK 8) and build-vaadin23 (JDK 11) jobs, because dev-nuke-maven-plugin 1.0.0 requires Java 17:

[ERROR] Required Java version 17 is not met by current version: 1.8.0_504

Blocked by FlowingCode/dev-nuke-maven-plugin#4.