Fix submodule refs to point to the correct REL_X_STABLE_neon branches (#8910)

Commit cfa45ff5ee (PR #8860) updated the vendor/postgres submodules, but
didn't use the same commit SHAs that were pushed as the corresponding
REL_*_STABLE_neon branches in the postgres repository. The contents were
the same, but the REL_*_STABLE_neon branches pointed to squashed
versions of the commits, whereas the SHAs used in the submodules
referred to the pre-squash revisions.

Note: The vendor/postgres-v14 submodule still doesn't match with the tip
of REL_14_STABLE_neon branch, because there has been one more commit on
that branch since then. That's another confusion which we should fix,
but let's do that separately. This commit doesn't change the code that
gets built in any way, only changes the submodule references to point to
the correct SHAs in the REL_*_STABLE_neon branch histories, rather than
some detached commits.

vendor/revisions.json

@@ -1,14 +1,14 @@
 {
   "v16": [
     "16.4",
-    "95132feffe277ce84309d93a42e9aadfd2cb0437"
+    "6e9a4ff6249ac02b8175054b7b3f7dfb198be48b"
   ],
   "v15": [
     "15.8",
-    "8aa1ded7726d416ac8e02600aad387a353478fc7"
+    "49d5e576a56e4cc59cd6a6a0791b2324b9fa675e"
   ],
   "v14": [
     "14.13",
-    "48388a5b597c81c09e28c016650a7156b48717a1"
+    "7602e907ab30f16188bebfd66b8f297c2889d339"
   ]
 }