build: update devdependency http-proxy-middleware to ~0.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
http-proxy-middleware	devDependencies	minor	~0.18.0 -> ~0.21.0

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Release Notes

chimurai/http-proxy-middleware

### [`v0.21.0`](https://togithub.com/chimurai/http-proxy-middleware/blob/master/CHANGELOG.md#v0210-httpsgithubcomchimuraihttp-proxy-middlewarereleasestagv0210) [Compare Source](https://togithub.com/chimurai/http-proxy-middleware/compare/v0.20.0...v0.21.0) - feat(http-proxy): bump to v1.18.0 - feat: async router ([#​379](https://togithub.com/chimurai/http-proxy-middleware/issues/379)) ([LiranBri](https://togithub.com/LiranBri)) - feat(typescript): types support ([#​369](https://togithub.com/chimurai/http-proxy-middleware/pull/369)) - feat: async pathRewrite ([#​397](https://togithub.com/chimurai/http-proxy-middleware/pull/397)) ([rsethc](https://togithub.com/rsethc)) ### [`v0.20.0`](https://togithub.com/chimurai/http-proxy-middleware/blob/master/CHANGELOG.md#v0200-httpsgithubcomchimuraihttp-proxy-middlewarereleasestagv0200) [Compare Source](https://togithub.com/chimurai/http-proxy-middleware/compare/v0.19.2...v0.20.0) - fix(ws): concurrent websocket requests do not get upgraded ([#​335](https://togithub.com/chimurai/http-proxy-middleware/issues/335)) - chore: drop node 6 (BREAKING CHANGE) - chore: update to micromatch@4 ([BREAKING CHANGE](https://togithub.com/micromatch/micromatch/blob/master/CHANGELOG.md#​400---2019-03-20)) - chore: update dev dependencies - refactor: migrate to typescript ([#​328](https://togithub.com/chimurai/http-proxy-middleware/pull/328)) - feat(middleware): Promise / async support ([#​328](https://togithub.com/chimurai/http-proxy-middleware/pull/328/files#diff-7890bfeb41abb0fc0ef2670749c84077R50)) - refactor: remove legacy options `proxyHost` and `proxyTable` (BREAKING CHANGE) ### [`v0.19.2`](https://togithub.com/chimurai/http-proxy-middleware/releases/v0.19.2) [Compare Source](https://togithub.com/chimurai/http-proxy-middleware/compare/v0.19.1...v0.19.2) - chore(deps): http-proxy 1.18.1 ### [`v0.19.1`](https://togithub.com/chimurai/http-proxy-middleware/blob/master/CHANGELOG.md#v0191-httpsgithubcomchimuraihttp-proxy-middlewarereleasestagv0191) [Compare Source](https://togithub.com/chimurai/http-proxy-middleware/compare/v0.19.0...v0.19.1) - fix(log): handle case when error code is missing ([#​303](https://togithub.com/chimurai/http-proxy-middleware/pull/303)) ### [`v0.19.0`](https://togithub.com/chimurai/http-proxy-middleware/blob/master/CHANGELOG.md#v0190-httpsgithubcomchimuraihttp-proxy-middlewarereleasestagv0190) [Compare Source](https://togithub.com/chimurai/http-proxy-middleware/compare/v0.18.0...v0.19.0) - feat(http-proxy): bump to v1.17.0 ([#​261](https://togithub.com/chimurai/http-proxy-middleware/pull/261))

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