Fix names of UBI-based Docker image build contexts

This commit fixes the name of the UBI-based Docker image build contexts
to include "7" (to set us up for the future where we are likely to have
a ubi8-based image).

distribution/docker/oss-ubi-docker-build-context/build.gradle

@@ -3,7 +3,7 @@ apply plugin: 'base'
 task buildOssUbiDockerBuildContext(type: Tar) {
     extension = 'tar.gz'
     compression = Compression.GZIP
-    archiveClassifier = "oss-ubi-docker-build-context"
+    archiveClassifier = "oss-ubi7-docker-build-context"
     archiveBaseName = "elasticsearch"
     with dockerBuildContext(true, true, false)
 }

distribution/docker/ubi-docker-build-context/build.gradle

@@ -3,7 +3,7 @@ apply plugin: 'base'
 task buildUbiDockerBuildContext(type: Tar) {
     extension = 'tar.gz'
     compression = Compression.GZIP
-    archiveClassifier = "ubi-docker-build-context"
+    archiveClassifier = "ubi7-docker-build-context"
     archiveBaseName = "elasticsearch"
     with dockerBuildContext(false, true, false)
 }