azurerm_mysql_flexible_server - add SKU B_Standard_B1s back (#29909)

[BUG] * `azurerm_mysql_flexible_server` - reverted a change made to the validation of the `sku_name` property that caused errors for existing resources

Author: neil-yechenwei

internal/services/mysql/validate/flexible_server_sku_name.go

@@ -16,7 +16,7 @@ func FlexibleServerSkuName(i interface{}, k string) (warnings []string, errors [
 	}
 
 	// See all available sku names from https://docs.microsoft.com/en-us/azure/mysql/flexible-server/concepts-compute-storage#compute-tiers-size-and-server-types
-	if !regexp.MustCompile(`^(B_(Standard_B(1m|2|2m|4m|8m|12m|16m|20m)s))|(GP_(Standard_D(2|4|8|16|32|48|64)ds_v4)|(Standard_D(2|4|8|16|32|48|64)ads_v5))|(MO_((Standard_E(2|4|8|16|20|32|48|64|80i)ds_v4)|(Standard_E(2|2a|4|4a|8|8a|16|16a|20|20a|32|32a|48|48a|64|64a|96|96a)ds_v5)))$`).MatchString(v) {
+	if !regexp.MustCompile(`^(B_(Standard_B(1|1m|2|2m|4m|8m|12m|16m|20m)s))|(GP_(Standard_D(2|4|8|16|32|48|64)ds_v4)|(Standard_D(2|4|8|16|32|48|64)ads_v5))|(MO_((Standard_E(2|4|8|16|20|32|48|64|80i)ds_v4)|(Standard_E(2|2a|4|4a|8|8a|16|16a|20|20a|32|32a|48|48a|64|64a|96|96a)ds_v5)))$`).MatchString(v) {
 		errors = append(errors, fmt.Errorf("%q is not a valid sku name, got %v", k, v))
 		return
 	}