Migrate from CircleCI to GitHub Actions for testing and releases, enable linting and changelog (#866)

Reference: #794

All testing and linting workflows verified as successful via:

```shell
act -s GITHUB_TOKEN=$GITHUB_TOKEN pull_request
```

Author: bflad

.changelog-note.tmpl

@@ -0,0 +1,3 @@
+{{- define "note" -}}
+{{.Body}} ([#{{- .Issue -}}](https://github.com/hashicorp/terraform-plugin-sdk/issues/{{- .Issue -}}))
+{{- end -}}

.changelog.tmpl

@@ -0,0 +1,42 @@
+{{- if .NotesByType.note -}}
+NOTES:
+{{range .NotesByType.note -}}
+* {{ template "note" .}}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.deprecation -}}
+DEPRECATIONS:
+{{range .NotesByType.deprecation -}}
+* {{ template "note" .}}
+{{ end -}}
+{{- end -}}
+
+{{- if index .NotesByType "breaking-change" -}}
+BREAKING CHANGES:
+{{range index .NotesByType "breaking-change" -}}
+* {{ template "note" .}}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.feature }}
+FEATURES:
+{{range .NotesByType.feature | sort -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.enhancement }}
+ENHANCEMENTS:
+{{range .NotesByType.enhancement | sort -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+
+{{- if .NotesByType.bug }}
+BUG FIXES:
+{{range .NotesByType.bug -}}
+* {{ template "note" . }}
+{{ end -}}
+{{- end -}}
+