Remove custom bundler

[jsvd]

Instead of relying on external bundler because JRuby flutuated between including and not including bundler for a while, now that bundler is part of rubygems, unless there's a CVE afecting the shipped version of bundler there's no need to have a custom one any more.
This simplifies our code and bootstrapping tasks, so this PR removes the need to install bundler during bootstrap.

As side effect this PR reintroduces the older patches to not go OOM during plugin docs generation.

TODO: find a way to not require installing bundler for the QA tests. We can deal with that in a follow up PR

[jsvd]

I have started a set of tests in CI for this remove_custom_bundler branch, that otherwise would only be run at merge time:

• windows compatibility tests ✅
• acceptance tests ✅
• docker acceptance tests ✅

[yaauie]

I am wary of running integration specs with a different bundler than we use at runtime, since the integration tests include coverage for installing and updating plugins which will use the bundler on the load path. This could lead to integration specs passing or failing independently of the actual behavior of our packaged artifacts.

[jsvd]

PR ready for a new review. I've removed the need to have a separate bundler for integration tests.
The PR now contains two commits:

• afb6363 reintroduces the patches needed for bundler < 2.4 to not go OOM during docs generation, and uses the right bundler classes for tests
• b98846d removes the need for an external bundler for all tasks by using JRuby's

[jsvd]

jenkins test this please

[yaauie]

Fantastic. I have left one note about guarding the DepProxy patch so that we remove or reimplement it when it no longer applies.

[jsvd]

jenkins test this please

[yaauie]

Fantastic. LGTM 👍🏼