Preferred file format for templates is `.tftpl`

[Bharath KKB]

Per Hashicorp documentation the preferred file format for templates is .tftpl

That will help avoid any confusion re: the double $'s

Originally posted by @rogerthatdev in #143 (comment)

[Roger Martinez]

File in question: eventarc_workflows/workflow.yaml

[Bharath KKB]

https://cloud.google.com/docs/terraform/best-practices-for-terraform#static-files

[Katie McLaughlin]

This appears to have been addressed as part of #350.

As of now, there doesn't appear template files in use in this repo, so nothing left to change.