[Security Solution] Force upgrading to target version for "Reference URLs" when base version is missing (#214161)

**Resolves: https://github.com/elastic/kibana/issues/214171**

## Summary

This PR fixes an issue with "references" field in -AB situations. When
the base version is missing we try to merge current and target arrays
which leads to old and potentially broken URLs from the current version
ending up in the result.

Now the behaviour is changed to always force merged version to be equal
to target (which always has correct URLs).

x-pack/solutions/security/plugins/security_solution/server/lib/detection_engine/prebuilt_rules/logic/diff/calculation/calculate_rule_fields_diff.ts

@@ -227,7 +227,7 @@ const commonFieldsDiffAlgorithms: FieldsDiffAlgorithmsFor<DiffableCommonFields>
   risk_score: numberDiffAlgorithm,
   risk_score_mapping: simpleDiffAlgorithm,
   references: createScalarArrayDiffAlgorithm({
-    missingBaseVersionStrategy: ScalarArrayDiffMissingBaseVersionStrategy.Merge,
+    missingBaseVersionStrategy: ScalarArrayDiffMissingBaseVersionStrategy.UseTarget,
   }),
   false_positives: simpleDiffAlgorithm,
   threat: simpleDiffAlgorithm,

x-pack/test/security_solution_api_integration/test_suites/detections_response/rules_management/prebuilt_rules/prebuilt_rule_customization/customization_enabled/diffable_rule_fields/common_fields/references.ts

@@ -297,11 +297,11 @@ export function referencesField({ getService }: FtrProviderContext): void {
             ruleUpgradeAssets,
             diffableRuleFieldName: 'references',
             expectedDiffOutcome: ThreeWayDiffOutcome.MissingBaseCanUpdate,
-            isMergableField: true,
+            isMergableField: false,
             expectedFieldDiffValues: {
               current: ['http://url-3'],
               target: ['http://url-1', 'http://url-2'],
-              merged: ['http://url-3', 'http://url-1', 'http://url-2'],
+              merged: ['http://url-1', 'http://url-2'],
             },
           },
           getService