[SGTM User Table] Deprecate SGTM's sync_users lambda, its code, and s…

…upporting resources (#166)

### Summary

Rely solely on the S3 mapping at the
`github_usernames_to_asana_gids_s3_path` variable. This simplifies
SGTM's implementation significantly since we no longer have to pull user
information from an Asana project.

This PR should only be merged once #165 has been merged in for a few
business days, and we've confirmed that no issues are arising and no
warnings about missing usernames are in the logs.

Asana tasks:
https://app.asana.com/0/1149418478823393/1204738744867777
https://app.asana.com/0/1200487107890647/1207040604662790/f


Relevant deployment: 

CC: @suzyng83209 @prebeta @vn6 @mattbryant-asana @skeggse
@Asana/continuous-integration

### Test Plan



### Risks



Pull Request: #166



Pull Request synchronized with [Asana
task](https://app.asana.com/0/0/1207061604941776)

---------

Co-authored-by: Harshita Gupta <harshita-gupta@users.noreply.github.com>

Allow SGTM to be setup with Terraform Cloud (#164)

SGTM has only used the s3-backed terraform backend with terragrunt for a
while. Asana is moving to using just Terraform Cloud so we want to
enable SGTM to move to TFC as well.

Changes:
* Created a new generate block with conditionals depending on which
remote backend wants to use. This should generate a well-formatted
`backend.tf`
* Still kept the old pathway for s3 remote backend
* Added 3 new TF vars to define the backend configuration
* Updated docs and instructions for deployment


Pull Request synchronized with [Asana
task](https://app.asana.com/0/0/1206682923832152)