Add Elasticsearch 7.4.3 to BWC test matrix

.ci/bwcVersions

@@ -11,6 +11,7 @@ BWC_VERSION:
   - "7.4.0"
   - "7.4.1"
   - "7.4.2"
+  - "7.4.3"
   - "7.5.0"
   - "7.6.0"
   - "8.0.0"