Rename GitHub repo from bazel-springboot-rule to rules_spring

[plaird]

This is a necessary final step for #30 The Git repo naming convention dictated by Bazel for public rules is rules_technology, where technology for our case is Spring. So our repo should be rules_spring. (note the underscore not a hyphen).

Up until recently, our scope was just springboot, so the rename could be rules_springboot. But if we implement #93 (SpringNative) we cover more than just Spring Boot in Bazel. So naming our repo rules_spring seems to be the right long term move.

I have done some local testing using a test repo, and here is what I found when we rename this repo:

• anyone using the old name over HTTP will get a 301 redirect to the repo with the new name
• Bazel http_repository rule follows redirects transparently
• Bazel git_repository rule follows redirects transparently if you use http as the protocol
• Bazel git_repository rule follows redirects transparently if you use ssh/git as the protocol
• browsers, of course, follow redirects transparently

Therefore, the GitHub repo rename should be transparent to all builds and existing documentation links.