chore: update circleci badge [backport 2.0] (#7027)

Backport fec32fb from #7018 to 2.0.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Co-authored-by: Tahir H. Butt <[email protected]>

Author: github-actions[bot]

README.md

@@ -1,6 +1,6 @@
 # `ddtrace`
 
-[![CircleCI](https://circleci.com/gh/DataDog/dd-trace-py/tree/1.x.svg?style=svg)](https://circleci.com/gh/DataDog/dd-trace-py/tree/1.x)
+[![CircleCI](https://circleci.com/gh/DataDog/dd-trace-py/tree/2.x.svg?style=svg)](https://circleci.com/gh/DataDog/dd-trace-py/tree/2.x)
 [![PypiVersions](https://img.shields.io/pypi/v/ddtrace.svg)](https://pypi.org/project/ddtrace/)
 [![Pyversions](https://img.shields.io/pypi/pyversions/ddtrace.svg?style=flat)](https://pypi.org/project/ddtrace/)
 
@@ -23,4 +23,4 @@ To get started as a contributor, see [the contributing docs](https://ddtrace.rea
 
 [setup docs]: https://docs.datadoghq.com/tracing/setup/python/
 [api docs]: https://ddtrace.readthedocs.io/
-[visualization docs]: https://docs.datadoghq.com/tracing/visualization/
+[visualization docs]: https://docs.datadoghq.com/tracing/visualization/