[#226] replace findbugs with spotbugs

[bmarwell]

• redefine profiles
• update CI
  • verify phase for ITs and spotbugs, as they are the default phases for both plugins. Do not deviate from sane defaults if not needed (convention over configuration)
• update README.md to reflect the updated contribution guidelines (run verify with both profiles)

fixes #226

Checks from GH Actions are expected to fail due to spotbugs issues.

[bmarwell]

LGTM, fails as expected: https://github.com/reficio/p2-maven-plugin/pull/229/checks?check_run_id=1969400518#step:5:2466