Incorrect Regex for resourceGroupName in WebApps

This would cause incorrect SDK code for the the validation.

The resource group name should follow the pattern defined in https://docs.microsoft.com/en-us/rest/api/resources/resourcegroups/createorupdate

https://github.com/Azure/azure-rest-api-specs/blob/270ba8923ce458af2b59cd66b008355676bccd49/specification/web/resource-manager/Microsoft.Web/stable/2021-02-01/WebApps.json#L26126

Start at specification/web/resource-manager/Microsoft.Web/stable/2021-02-01/WebApps.json around line 26126 and compare the resourceGroupName regex with the linked Azure Resource Groups Create or Update documentation. Update the specification so generated SDK validation follows that documented pattern, then verify the spec remains valid and the generated validation reflects the corrected constraint.

Written by the indexing model from the issue text.