Use underscores in Terraform file names instead of dashes (#828)

## Ticket

Resolves #739 

## Changes

- Swapped dashes for underscores in the 2 remaining files that were
inconsistent
- Added info to style guide

## Testing

`find . -type f -name '*.tf' | awk -F/ '{print $NF}' | grep -- '-'`

[Platform Test PR](navapbc/platform-test#152)

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- Service endpoint:
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.github/workflows/send-system-notification.yml

@@ -4,7 +4,7 @@ on:
   workflow_dispatch:
     inputs:
       channel:
-        description: "Name of channel to use. Must be defined in /infra/project-config/system-notifications.tf"
+        description: "Name of channel to use. Must be defined in /infra/project-config/system_notifications.tf"
         required: true
         type: string
       message:

docs/infra/style-guide.md

@@ -7,6 +7,7 @@
   - [Terraform code style](#terraform-code-style)
     - [Exceptions and additions to Hashicorp's Terraform style guide](#exceptions-and-additions-to-hashicorps-terraform-style-guide)
       - [Modules](#modules)
+      - [File Names](#file-names)
       - [Variables](#variables)
       - [.gitignore](#gitignore)
       - [Integration and unit testing](#integration-and-unit-testing)
@@ -28,6 +29,11 @@ Here are some exceptions (and additions) to Hashicorp's Terraform style guide.
 - [Use shared configuration](/docs/infra/module-dependencies.md) instead of the [tfe_outputs data source](https://registry.terraform.io/providers/hashicorp/tfe/latest/docs/data-sources/outputs) to share state between two state files.
 - Use underscores instead of dashes in file names and module names.
 
+#### File Names
+
+- Separate words in filenames with underscores (\_) instead of dashes (-), e.g., main.tf, output_variables.tf.
+- Use lowercase letters to avoid case sensitivity issues.
+
 #### Variables
 
 - Include additional type information in string variable names to clarify the value being stored. For example, use `access_policy_arn` instead of `access_policy`. Common examples of suffixes include: `_id`, `_arn`, and `_name`.

docs/infra/system-notifications.md

@@ -4,4 +4,4 @@ The project sends notifications as part of CI/CD workflows to notify the team ab
 
 ## System notifications configuration
 
-The configuration for system notifications is defined in the application's [project-config module](/infra/project-config/). The [system-notifications.tf](/infra/project-config/system-notifications.tf) file defines one or more notification channels that CI/CD workflows can send notifications to. Each channel can use a different notification type. Currently, Slack is the only supported notification type.
+The configuration for system notifications is defined in the application's [project-config module](/infra/project-config/). The [system_notifications.tf](/infra/project-config/system_notifications.tf) file defines one or more notification channels that CI/CD workflows can send notifications to. Each channel can use a different notification type. Currently, Slack is the only supported notification type.