Update dependency express to v4.21.0 (#124)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [express](http://expressjs.com/)
([source](https://redirect.github.com/expressjs/express)) | [`4.20.0` ->
`4.21.0`](https://renovatebot.com/diffs/npm/express/4.20.0/4.21.0) |
[![age](https://developer.mend.io/api/mc/badges/age/npm/express/4.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/express/4.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/express/4.20.0/4.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/express/4.20.0/4.21.0?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>expressjs/express (express)</summary>

###
[`v4.21.0`](https://redirect.github.com/expressjs/express/releases/tag/4.21.0)

[Compare
Source](https://redirect.github.com/expressjs/express/compare/4.20.0...4.21.0)

#### What's Changed

- Deprecate `"back"` magic string in redirects by
[@&#8203;blakeembrey](https://redirect.github.com/blakeembrey) in
[expressjs/express#5935
- [email protected] by
[@&#8203;wesleytodd](https://redirect.github.com/wesleytodd) in
[expressjs/express#5954
- fix(deps): [email protected] by
[@&#8203;wesleytodd](https://redirect.github.com/wesleytodd) in
[expressjs/express#5951
- Upgraded dependency qs to 6.13.0 to match qs in body-parser by
[@&#8203;agadzinski93](https://redirect.github.com/agadzinski93) in
[expressjs/express#5946

#### New Contributors

- [@&#8203;agadzinski93](https://redirect.github.com/agadzinski93) made
their first contribution in
[expressjs/express#5946

**Full Changelog**:
expressjs/express@4.20.0...4.21.0

</details>

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