change capability name to keep consistent

Author: romulets

x-pack/plugin/esql/qa/testFixtures/src/main/resources/date.csv-spec

@@ -1838,7 +1838,7 @@ y:date                   | c:long
 ;
 
 dayNameRowTest
-required_capability: day_name_function
+required_capability:fn_day_name
 // tag::docsDayName[]
 ROW dt = to_datetime("1953-09-02T00:00:00.000Z")
 | EVAL weekday = DAY_NAME(dt);
@@ -1849,7 +1849,7 @@ dt:date   | weekday:keyword
 ;
 
 dayNameSimple
-required_capability: day_name_function
+required_capability:fn_day_name
 from employees 
 | sort emp_no 
 | keep emp_no, hire_date
@@ -1861,7 +1861,7 @@ emp_no:integer | hire_date:date             | day:keyword
 ;
 
 dayNameTruncate
-required_capability: day_name_function
+required_capability:fn_day_name
 FROM employees
 | EVAL a = hire_date
 | RENAME hire_date as b
@@ -1882,7 +1882,7 @@ y:date                   | c:long   | week_day:keyword
 ;
 
 dayNameNestedCall
-required_capability: day_name_function
+required_capability:fn_day_name
 from employees 
 | sort emp_no 
 | eval first_day_of_month = day_name(date_trunc(1 month, hire_date))
@@ -1903,7 +1903,7 @@ emp_no:integer | hire_date:datetime       | first_day_of_month:keyword
 ;
 
 dayNameNestedCall
-required_capability: day_name_function
+required_capability:fn_day_name
 from employees 
 | sort emp_no desc
 | eval first_day_of_month = to_upper(day_name(date_trunc(1 month, hire_date)))
@@ -1924,7 +1924,7 @@ emp_no:integer | hire_date:datetime       | first_day_of_month:keyword
 ;
 
 dayNameNull
-required_capability: day_name_function
+required_capability:fn_day_name
 from employees | where emp_no == 10040 | eval x = day_name(birth_date) | keep emp_no, birth_date, hire_date, x;
 
 emp_no:integer | birth_date:date                     | hire_date:date             | x:keyword

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -235,6 +235,11 @@ public enum Cap {
          */
         FN_SCALB,
 
+        /**
+         * Support for function DAY_NAME
+         */
+        FN_DAY_NAME,
+
         /**
          * Fixes for multiple functions not serializing their source, and emitting warnings with wrong line number and text.
          */
@@ -1335,12 +1340,7 @@ public enum Cap {
         /**
          * Support correct counting of skipped shards.
          */
-        CORRECT_SKIPPED_SHARDS_COUNT,
-
-        /**
-         * Supports DAY_NAME function
-         */
-        DAY_NAME_FUNCTION;
+        CORRECT_SKIPPED_SHARDS_COUNT;
 
         private final boolean enabled;