[DPB] update flex counter yang model, fix BUFFER_POOL_WATERMARK (#6178)

Looks like we have redefinition of BUFFER_POOL_WATERMARK_KEY to BUFFER_POOL_WATERMARK, so DPB fall.
https://github.com/Azure/sonic-swss/blob/c7ee75f0b92e3bbbd34938fb6ceca50f0491b5f9/orchagent/flexcounterorch.cpp#L18

#define BUFFER_POOL_WATERMARK_KEY   "BUFFER_POOL_WATERMARK"


unordered_map<string, string> flexCounterGroupMap =
{
    {"PORT", PORT_STAT_COUNTER_FLEX_COUNTER_GROUP},
    {"PORT_RATES", PORT_RATE_COUNTER_FLEX_COUNTER_GROUP},
    {"PORT_BUFFER_DROP", PORT_STAT_COUNTER_FLEX_COUNTER_GROUP},
    {"QUEUE", QUEUE_STAT_COUNTER_FLEX_COUNTER_GROUP},
    {"PFCWD", PFC_WD_FLEX_COUNTER_GROUP},
    {"QUEUE_WATERMARK", QUEUE_WATERMARK_STAT_COUNTER_FLEX_COUNTER_GROUP},
    {"PG_WATERMARK", PG_WATERMARK_STAT_COUNTER_FLEX_COUNTER_GROUP},
    {BUFFER_POOL_WATERMARK_KEY, BUFFER_POOL_WATERMARK_STAT_COUNTER_FLEX_COUNTER_GROUP},

src/sonic-yang-models/tests/yang_model_tests/yangTest.json

@@ -850,7 +850,7 @@
                 "PORT_BUFFER_DROP": {
                     "FLEX_COUNTER_STATUS": "enable"
                 },
-                "BUFFER_POOL_WATERMARK_KEY": {
+                "BUFFER_POOL_WATERMARK": {
                     "FLEX_COUNTER_STATUS": "enable"
                 },
                 "QUEUE": {
@@ -1689,7 +1689,7 @@
             "PORT_BUFFER_DROP": {
                 "FLEX_COUNTER_STATUS": "enable"
             },
-            "BUFFER_POOL_WATERMARK_KEY": {
+            "BUFFER_POOL_WATERMARK": {
                 "FLEX_COUNTER_STATUS": "enable"
             },
             "QUEUE": {

src/sonic-yang-models/yang-models/sonic-flex_counter.yang

@@ -27,7 +27,7 @@ module sonic-flex_counter {
 
             /* below are in alphabetical order */
 
-            container BUFFER_POOL_WATERMARK_KEY {
+            container BUFFER_POOL_WATERMARK {
                 /* BUFFER_POOL_WATERMARK_STAT_COUNTER_FLEX_COUNTER_GROUP */
                 leaf FLEX_COUNTER_STATUS {
                     type flex_status;