fix!: Remove unnecessary breaking changes when upgrading to v11.0.0+

[thatcoleyouknow]

PR https://github.com/terraform-google-modules/terraform-google-vm/pull/330 introduced a breaking change for existing templates that were deployed with this module. For some consumers, this creates an unnecessary redeployment of all downstream instances that may come at great costs. This is an optional argument and I feel this should be treated as such with a dynamic block. This PR enables consumers that are upgrading from version <10.0.0 to do so without recreating their template and downstream instances while also allowing consumers that are using version >10.0.0 to avoid recreating their instances and templates by simply setting the argument to match their preferred value.

Note: I made all of these changes in the GitHub editor, not my IDE, so if I missed something, please let me know and I'll get it fixed.