Fix Postgres build on macOS (#11442)

## Problem
Postgres build fails with the following error on macOS:

```
/Users/bayandin/work/neon//vendor/postgres-v14/src/port/snprintf.c:424:27: error: 'strchrnul' is only available on macOS 15.4 or newer [-Werror,-Wunguarded-availability-new]
  424 |                         const char *next_pct = strchrnul(format + 1, '%');
      |                                                ^~~~~~~~~
/Users/bayandin/work/neon//vendor/postgres-v14/src/port/snprintf.c:376:14: note: 'strchrnul' has been marked as being introduced in macOS 15.4 here, but the deployment target is macOS 15.0.0
  376 | extern char *strchrnul(const char *s, int c);
      |              ^
/Users/bayandin/work/neon//vendor/postgres-v14/src/port/snprintf.c:424:27: note: enclose 'strchrnul' in a __builtin_available check to silence this warning
  424 |                         const char *next_pct = strchrnul(format + 1, '%');
      |                                                ^~~~~~~~~
  425 |
  426 |                         /* Dump literal data we just scanned over */
  427 |                         dostr(format, next_pct - format, target);
  428 |                         if (target->failed)
  429 |                                 break;
  430 |
  431 |                         if (*next_pct == '\0')
  432 |                                 break;
  433 |                         format = next_pct;
      |
1 error generated.
```

## Summary of changes
- Update Postgres fork to include changes from
6da2ba1d8a

Corresponding Postgres PRs:
- https://github.com/neondatabase/postgres/pull/608
- https://github.com/neondatabase/postgres/pull/609
- https://github.com/neondatabase/postgres/pull/610
- https://github.com/neondatabase/postgres/pull/611

vendor/revisions.json

@@ -1,18 +1,18 @@
 {
   "v17": [
     "17.4",
-    "7ec41bf6cd92a4af751272145fdd590270c491da"
+    "c9e4ff5a38907acd71107634055bf2609aba43a5"
   ],
   "v16": [
     "16.8",
-    "26c7d3f6de6f361c8923bb80d7563853b4a04958"
+    "746bd9ffe5c29bce030eaea1031054057f3c5d45"
   ],
   "v15": [
     "15.12",
-    "4ac24a747cd897119ce9b20547b3b04eba2cacbd"
+    "23708b3aca9adf163aa0973eb63d9afc0e4a04c3"
   ],
   "v14": [
     "14.17",
-    "bce3e48d8a72e70e72dfee1b7421fecd0f1b00ac"
+    "8cca70c22e2894dd4645f9a940086ac437b0a11b"
   ]
 }