convert databaseVersion enum to string (GoogleCloudPlatform#12687)

Co-authored-by: Minoru, Kawamoto <[email protected]>

Author: k12u

mmv1/products/sql/SourceRepresentationInstance.yaml

@@ -66,20 +66,10 @@ properties:
     required: false
     default_from_api: true
   - name: 'databaseVersion'
-    type: Enum
+    type: String
     description: |
-      The MySQL version running on your source database server.
+      The MySQL, PostgreSQL or SQL Server (beta) version to use. Supported values include MYSQL_5_6, MYSQL_5_7, MYSQL_8_0, MYSQL_8_4, POSTGRES_9_6, POSTGRES_10, POSTGRES_11, POSTGRES_12, POSTGRES_13, POSTGRES_14, POSTGRES_15, POSTGRES_16, POSTGRES_17. Database Version Policies includes an up-to-date reference of supported versions.
     required: true
-    enum_values:
-      - 'MYSQL_5_6'
-      - 'MYSQL_5_7'
-      - 'MYSQL_8_0'
-      - 'POSTGRES_9_6'
-      - 'POSTGRES_10'
-      - 'POSTGRES_11'
-      - 'POSTGRES_12'
-      - 'POSTGRES_13'
-      - 'POSTGRES_14'
   - name: 'onPremisesConfiguration'
     type: NestedObject
     description: |