Drop support for Bazel 1.x

[plaird]

This is a marker Issue. Not much work is needed aside from some small doc edits, a release note, and update the Bazelisk .bazelversion file. This is filed as an issue mostly so we can be intentional about when to do this.

Thoughts:

• Spring Boot rule release 1.1.0 will continue to support Bazel 1.2.1.
• We will try to implement as many features/bug fixes as possible before dropping Bazel 1.2.1 support so that users can remain on 1.2.1 if they choose and be happy
• Issue Use Bazel configured java_runtime_version for bazel run invocations (deferred) #16 is the one I am thinking of that could encourage us to drop 1.2.1 sooner rather than later
• Consider timeline for dropping Bazel 2.x support. I don't think it is as urgent, but that is just guessing.

[plaird]

I don't know if rules_spring still works on Bazel 1.x, but no one should be running it.