chore(project): remove CircleCI configuration (#15339)

Because

* The CircleCI-to-GHA migration (EXP-6320) is complete and all jobs have
been running in GHA for the past 24+ hours alongside CircleCI
* Branch protections have been switched to require the GHA checks
* CircleCI is no longer referenced by any required workflow

This commit

* Deletes \`.circleci/config.yml\` and
\`.circleci/scripts/create_jwt.py\`
* Drops \`CIRCLE_PULL_REQUEST\` / \`CIRCLE_BRANCH\` env vars from
\`docker-compose-test.yml\`
* Drops \`CIRCLECI\` pass-through from the integration test make target
and removes the \`CIRCLECI\` gate in the nimbus conftest cleanup
* Drops the \`CIRCLE_BUILD_URL\` fallback from
\`scripts/echo_version_json.sh\`
* Swaps the CircleCI status badge in \`README.md\` for the GHA check
workflow badge, and updates remaining references from
\`.circleci/config.yml\` to \`.github/workflows/\`

Fixes #15216

Author: jaredlockhart