Upgrade findbugs to spotbugs for 1.9

(Backport of #615 to the 1.9 branch with some updates)

* Upgrade findbugs to spotbugs 3.1.6
* Rename all findbugs to spotbugs (with various capitalizations)
* Remove README.spotbugs/README.findbugs (contains incorrect info and not useful)
diff --git a/src/main/findbugs/exclude-filter.xml b/src/main/spotbugs/exclude-filter.xml
similarity index 100%
rename from src/main/findbugs/exclude-filter.xml
rename to src/main/spotbugs/exclude-filter.xml