Fix IndexLifecycleExplainResponse serialization version checks

This fixes the version checks to use 7.8.0 instead of 8.0.0 since the
changes in elastic#54825 were backported to the 7.x branch.

Author: andreidan

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/IndexLifecycleExplainResponse.java

@@ -194,7 +194,7 @@ public IndexLifecycleExplainResponse(StreamInput in) throws IOException {
                 isAutoRetryableError = null;
                 failedStepRetryCount = null;
             }
-            if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+            if (in.getVersion().onOrAfter(Version.V_7_8_0)) {
                repositoryName = in.readOptionalString();
                snapshotName = in.readOptionalString();
             } else {
@@ -240,7 +240,7 @@ public void writeTo(StreamOutput out) throws IOException {
                 out.writeOptionalBoolean(isAutoRetryableError);
                 out.writeOptionalVInt(failedStepRetryCount);
             }
-            if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+            if (out.getVersion().onOrAfter(Version.V_7_8_0)) {
                 out.writeOptionalString(repositoryName);
                 out.writeOptionalString(snapshotName);
             }