[Response Ops] [Alerting] Excluding ECS fields of type constant_keyword from ECS field map (#155075)

Fields mapped as `constant_keyword` can cause issues when used in the
alerts as data mapping where multiple types of sources are combined into
one index. These fields were previously excluded from the ECS field
mapping used by alerts as data. We included them because we wanted to
use ECS as closely as possible but it is causing downstream issues so
we'll continue excluding them until we decide we need them at some point
in the future.

## To verify:
1. Start ES & Kibana
2. Inspect the `.alerts-ecs-mappings` component template mapping and
verify there are no fields with type `constant_keyword`

packages/kbn-alerts-as-data-utils/src/field_maps/ecs_field_map.ts

@@ -9,8 +9,16 @@
 import { EcsFlat } from '@kbn/ecs';
 import { EcsMetadata, FieldMap } from './types';
 
+const EXCLUDED_TYPES = ['constant_keyword'];
+
 export const ecsFieldMap: FieldMap = Object.keys(EcsFlat).reduce((acc, currKey) => {
   const value: EcsMetadata = EcsFlat[currKey as keyof typeof EcsFlat];
+
+  // Exclude excluded types
+  if (EXCLUDED_TYPES.includes(value.type)) {
+    return acc;
+  }
+
   return {
     ...acc,
     [currKey]: {