Replace FindBugs with SpotBugs

Replace FindBugs with SpotBugs

SpotBugs [1] is the spiritual successor of FindBugs, carrying on from
the point where it left off with support of its community.

This is a backport of [1] which originally did the replacement on the
master branch. This change updates to the current latest version, so
that we can get the benefit of its checks when pushing changes to the
stable branches.

[1] https://spotbugs.github.io/
[2] https://git.eclipse.org/r/#/c/101312/

Change-Id: Ib73d56b5980b55f4d7e09d87abec3138cac3d3dc
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>