ci: Remove CircleCI configuration (#3065)

CI is now performed via Github Actions. Some notable Github Action
workflows that replace the CircleCI jobs are:

- test: replaces CCI build, test-api, test-sdk
- test-sql: replaces CCI test-sql
- make-gen-delta: replaces CCI make-gen-delta

See:
    https://github.com/hashicorp/boundary/actions/workflows/test.yml
    https://github.com/hashicorp/boundary/actions/workflows/test-sql.yml
    https://github.com/hashicorp/boundary/actions/workflows/make-gen-delta.yml
    https://docs.github.com/en/actions
    https://cli.github.com/manual/gh_run

.copywrite.hcl

@@ -5,7 +5,6 @@ project {
   copyright_year = 2023
 
   header_ignore = [
-    ".circleci/**",
     ".github/**",
     ".golangci.yml",
     ".release/linux/**",