Unmute FieldSortIT testSortMixedFieldTypes (elastic#138089)

Author: cbuescher

muted-tests.yml

@@ -371,9 +371,6 @@ tests:
 - class: org.elasticsearch.packaging.test.DockerTests
   method: test151MachineDependentHeapWithSizeOverride
   issue: https://github.com/elastic/elasticsearch/issues/123437
-- class: org.elasticsearch.search.sort.FieldSortIT
-  method: testSortMixedFieldTypes
-  issue: https://github.com/elastic/elasticsearch/issues/129445
 - class: org.elasticsearch.gradle.LoggedExecFuncTest
   method: failed tasks output logged to console when spooling true
   issue: https://github.com/elastic/elasticsearch/issues/119509