Fix tests broken because future not completed during cleanup (elastic#121782) (elastic#121817)

A future.actionGet was missing from the delete pipeline action execution in the test cleanup, causing all tests to fail intermittently. Also replace actionGet with safeGet.

(cherry picked from commit 0f6b80a)

# Conflicts:
#	muted-tests.yml
#	x-pack/plugin/migrate/src/internalClusterTest/java/org/elasticsearch/xpack/migrate/action/ReindexDatastreamIndexTransportActionIT.java

Author: parkertimmins

muted-tests.yml

@@ -531,9 +531,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.action.CrossClustersCancellationIT
   method: testCancelSkipUnavailable
   issue: https://github.com/elastic/elasticsearch/issues/121631
-- class: org.elasticsearch.xpack.migrate.action.ReindexDatastreamIndexTransportActionIT
-  method: testTimestamp0AddedIfMissing
-  issue: https://github.com/elastic/elasticsearch/issues/121745
 - class: org.elasticsearch.smoketest.DocsClientYamlTestSuiteIT
   method: test {yaml=reference/rest-api/common-options/line_102}
   issue: https://github.com/elastic/elasticsearch/issues/121748