Update Heroku API client

[mars]

Internal work item: [terraform-provider-heroku] Fir Foundation: update Heroku API Go client to latest schema.

Pull in heroku-go update for latest API schema, testing with the branch first.

To get CI acceptance tests passing,

1. updated GitHub runner Terraform installation workflow step
2. updated to current Go version
3. updated heroku-go API client usage where API schema changes over the past two-years broke existing usage
   1. Removed heroku_cert resource which relied on the sunset Heroku SSL Endpoint product
   2. Updated for new Formation DynoSize interface
   3. Fixed test Ruby app sources to successfully build across tests

[heroku-johnny]

Reviewed this PR with @mars and this LGTM!

• Need to bump the heroku-go client version