Enable BWC tests on FIPS (elastic#85142)

FIPS BWC tests were disabled because there was an intentional
change in FIPS behaviour in ES v7.11.0 which means that testing of
compatibility between versions older than that and versions newer than
that change had certain complexities.

Since 8.x no longer needs to be compatible with anything older than
7.17, we can enable these tests again

build.gradle

@@ -141,16 +141,6 @@ tasks.register("verifyVersions") {
 boolean bwc_tests_enabled = true
 // place a PR link here when committing bwc changes:
 String bwc_tests_disabled_issue = ""
-/*
- * FIPS 140-2 behavior was fixed in 7.11.0. Before that there is no way to run elasticsearch in a
- * JVM that is properly configured to be in fips mode with BCFIPS. For now we need to disable
- * all bwc testing in fips mode.
- */
-
-if ( BuildParams.inFipsJvm ) {
-  bwc_tests_enabled = false
-  bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/issues/66772"
-}
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")