[ES|QL] case only requires two parameters (elastic#179011)

## Summary

Part of elastic#177699

We had `case` marked as if it required three parameters when in reality
it only requires two.

<img width="600" alt="Screenshot 2024-03-19 at 4 23 29 PM"
src="https://github.com/elastic/kibana/assets/315764/45f7578a-e6ad-4ba9-b71a-05bb1978a384">

Note: we could consider testing these n-1 cases to prevent this kind of
bug in the future.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Marta Bondyra <4283304+mbondyra@users.noreply.github.com>

packages/kbn-monaco/src/esql/lib/ast/definitions/functions.ts

@@ -592,7 +592,7 @@ export const evalFunctionsDefinitions: FunctionDefinition[] = [
           { name: 'condition', type: 'boolean' },
           { name: 'value', type: 'any' },
         ],
-        minParams: 3,
+        minParams: 2,
         returnType: 'any',
         examples: [
           `from index | eval type = case(languages <= 1, "monolingual", languages <= 2, "bilingual", "polyglot")`,

packages/kbn-monaco/src/esql/lib/ast/validation/esql_validation_meta_tests.json

@@ -563,11 +563,11 @@
       "error": false
     },
     {
-      "query": "row var = case(true, \"a\", \"a\")",
+      "query": "row var = case(true, \"a\")",
       "error": false
     },
     {
-      "query": "row case(true, \"a\", \"a\")",
+      "query": "row case(true, \"a\")",
       "error": false
     },
     {
@@ -4871,11 +4871,11 @@
       "error": true
     },
     {
-      "query": "from a_index | eval var = case(booleanField, stringField, stringField)",
+      "query": "from a_index | eval var = case(booleanField, stringField)",
       "error": false
     },
     {
-      "query": "from a_index | eval case(booleanField, stringField, stringField)",
+      "query": "from a_index | eval case(booleanField, stringField)",
       "error": false
     },
     {