feat: avm/res/db-for-postgresql/flexible server (Azure#3088)

## Description

Added support for PostgeSQL version 16.

closes Azure#2203 
closes Azure#1310 

## Pipeline Reference

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## Type of Change

<!-- Use the checkboxes [x] on the options that are relevant. -->

- [ ] Update to CI Environment or utilities (Non-module affecting
changes)
- [X] Azure Verified Module updates:
- [ ] Bugfix containing backwards-compatible bug fixes, and I have NOT
bumped the MAJOR or MINOR version in `version.json`:
- [X] Someone has opened a bug report issue, and I have included "Closes
#{bug_report_issue_number}" in the PR description.
- [ ] The bug was found by the module author, and no one has opened an
issue to report it yet.
- [ ] Feature update backwards compatible feature updates, and I have
bumped the MINOR version in `version.json`.
- [ ] Breaking changes and I have bumped the MAJOR version in
`version.json`.
  - [X] Update to documentation

## Checklist

- [X] I'm sure there are no other open Pull Requests for the same
update/change
- [X] I have run `Set-AVMModule` locally to generate the supporting
module files.
- [X] My corresponding pipelines / checks run clean and green without
any errors or warnings

<!-- Please keep up to date with the contribution guide at
https://aka.ms/avm/contribute/bicep -->

avm/res/db-for-postgre-sql/flexible-server/README.md

@@ -1518,7 +1518,7 @@ PostgreSQL Server version.
 
 - Required: No
 - Type: string
-- Default: `'15'`
+- Default: `'16'`
 - Allowed:
   ```Bicep
   [
@@ -1527,6 +1527,7 @@ PostgreSQL Server version.
     '13'
     '14'
     '15'
+    '16'
   ]
   ```
 

avm/res/db-for-postgre-sql/flexible-server/main.bicep

@@ -89,9 +89,10 @@ param storageSizeGB int = 32
   '13'
   '14'
   '15'
+  '16'
 ])
 @description('Optional. PostgreSQL Server version.')
-param version string = '15'
+param version string = '16'
 
 @allowed([
   'Disabled'

avm/res/db-for-postgre-sql/flexible-server/main.json

@@ -6,7 +6,7 @@
     "_generator": {
       "name": "bicep",
       "version": "0.29.47.4906",
-      "templateHash": "11155893103946575130"
+      "templateHash": "6369286683210643119"
     },
     "name": "DBforPostgreSQL Flexible Servers",
     "description": "This module deploys a DBforPostgreSQL Flexible Server.",
@@ -413,13 +413,14 @@
     },
     "version": {
       "type": "string",
-      "defaultValue": "15",
+      "defaultValue": "16",
       "allowedValues": [
         "11",
         "12",
         "13",
         "14",
-        "15"
+        "15",
+        "16"
       ],
       "metadata": {
         "description": "Optional. PostgreSQL Server version."