Enable disabled rolling upgrade tests

These tests were disabled due to a bug introduced when
sequence-number-based recovery was introduced. This commit enables these
tests again.

Relates elastic/elasticsearch#4801

Original commit: elastic/x-pack-elasticsearch@e555bfec1a

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/20_security.yaml

@@ -1,8 +1,5 @@
 ---
 "Verify user and role in mixed cluster":
-  - skip:
-      version: " - 6.0.0"
-      reason: Temporary until test issue is resolved
   - do:
       headers:
         Authorization: "Basic bmF0aXZlX3VzZXI6Y2hhbmdlbWU="

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/20_security.yaml

@@ -1,8 +1,5 @@
 ---
 "Create user and role":
-  - skip:
-        version: " - 6.0.0"
-        reason: Temporary until test issue is resolved
   - do:
      cluster.health:
         wait_for_status: green

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/20_security.yaml

@@ -1,8 +1,5 @@
 ---
 "Verify user and role in upgraded cluster":
-  - skip:
-      version: " - 6.0.0"
-      reason: Temporary until test issue is resolved
   - do:
       headers:
         Authorization: "Basic bmF0aXZlX3VzZXI6Y2hhbmdlbWU="