provider: Bump Go version to 1.15 and standardize on .go-version file for GitHub Actions workflows

[bflad]

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Reference: https://github.com/hashicorp/terraform-provider-aws/blob/master/docs/MAINTAINING.md#go-default-version-update
Reference: https://golang.org/doc/go1.15

Release note for CHANGELOG:

provider: The default development, testing, and building of the Terraform AWS Provider binary is now done with Go 1.15. This version of Go now requires macOS 10.12 Sierra or later.

Provider development with Go 1.15 has been very stable the last few months.

Output from acceptance testing: Handled via daily acceptance testing

[YakDriver]

Cool!

[bflad]

This should be okay for the provider release process now.

[ghost]

This has been released in version 3.22.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!