fix: Set subnet batch size decorator (Azure#1771)

## Description
Rerunning the vnet module with multiple subnets over an existing vnet
resource can at times cause a 'another operation is in progress' error.
For this reason the batchSize decorator has been added for the
subsequent subnet child module call to minimise this error.

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Fixes Azure#1519
Closes Azure#1519
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## Type of Change

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- [ ] Update to CI Environment or utlities (Non-module effecting
changes)
- [x] Azure Verified Module updates:
- [x] 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

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---------

Co-authored-by: Rodney Almeida <[email protected]>
Co-authored-by: ChrisSidebotham-MSFT <[email protected]>

avm/res/network/virtual-network/main.bicep

@@ -167,6 +167,7 @@ resource virtualNetwork 'Microsoft.Network/virtualNetworks@2023-04-01' = {
 // You can safely remove the below child module (virtualNetwork_subnets) in your consumption of the module (virtualNetworks) to reduce the template size and duplication.
 //NOTE End  : ------------------------------------
 
+@batchSize(1)
 module virtualNetwork_subnets 'subnet/main.bicep' = [
   for (subnet, index) in subnets: {
     name: '${uniqueString(deployment().name, location)}-subnet-${index}'

avm/res/network/virtual-network/main.json

@@ -483,7 +483,9 @@
     "virtualNetwork_subnets": {
       "copy": {
         "name": "virtualNetwork_subnets",
-        "count": "[length(parameters('subnets'))]"
+        "count": "[length(parameters('subnets'))]",
+        "mode": "serial",
+        "batchSize": 1
       },
       "type": "Microsoft.Resources/deployments",
       "apiVersion": "2022-09-01",