GoReleaser was too old, update config options

Oops, I forgot `go mod tidy`

Bump GitHub Actions checkout/go/tf versions too

Bump issyl0/go-improvmx version

CHANGELOG: Prepare for 0.5.0.

- This'll include #9. 🎉

docs: Use the correct resource name in `improvmx_smtp_credential` docs

CHANGELOG: Create, add a description of all the versions

provider,resource_smtp_credential: Support configuring SMTP credentials

- https://improvmx.com/api/#smtp-list

resource_email_forward: Fix importing, set `alias_name` before reading

- The post-import plan for an email forward wouldn't come back clean as
  all the fields weren't set in the state.

README: Remove provider version

- If users copy/paste this without changing anything, they'll be on a super early version.