Adapt bwc serialization in FieldSortBuilder (#40095)

This change adapts the bwc version check for the new numeric_type option of the FieldSortBuilder.
Since this change needs to be backported to 7x, all bwc tests are temporary disabled
until https://github.com/elastic/elasticsearch/pull/40084 is merged.

Relates #38095

build.gradle

@@ -162,8 +162,8 @@ task verifyVersions {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = true
-final String bwc_tests_disabled_issue = "" /* place a PR link here when committing bwc changes */
+boolean bwc_tests_enabled = false
+final String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/40084" /* place a PR link here when committing bwc changes */
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")

server/src/main/java/org/elasticsearch/search/sort/FieldSortBuilder.java

@@ -128,7 +128,7 @@ public class FieldSortBuilder extends SortBuilder<FieldSortBuilder> {
         sortMode = in.readOptionalWriteable(SortMode::readFromStream);
         unmappedType = in.readOptionalString();
         nestedSort = in.readOptionalWriteable(NestedSortBuilder::new);
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (in.getVersion().onOrAfter(Version.V_7_1_0)) {
             numericType = in.readOptionalString();
         }
     }
@@ -143,7 +143,7 @@ public class FieldSortBuilder extends SortBuilder<FieldSortBuilder> {
         out.writeOptionalWriteable(sortMode);
         out.writeOptionalString(unmappedType);
         out.writeOptionalWriteable(nestedSort);
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_1_0)) {
             out.writeOptionalString(numericType);
         }
     }