build: update devdependency eslint to ~7.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
eslint (source)	~7.20.0 -> ~7.21.0

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Release Notes

eslint/eslint

### [`v7.21.0`](https://togithub.com/eslint/eslint/releases/v7.21.0) [Compare Source](https://togithub.com/eslint/eslint/compare/v7.20.0...v7.21.0) - [`3cd5440`](https://togithub.com/eslint/eslint/commit/3cd5440b94d5fa4f11a09f50b685f6150f0c2d41) Upgrade: [@​eslint/eslintrc](https://togithub.com/eslint/eslintrc) to 0.4.0 ([#​14147](https://togithub.com/eslint/eslint/issues/14147)) (Brandon Mills) - [`c0b8c71`](https://togithub.com/eslint/eslint/commit/c0b8c71df4d0b3f54b20587432d9133741985d5c) Upgrade: Puppeteer to 7.1.0 ([#​14122](https://togithub.com/eslint/eslint/issues/14122)) (Tim van der Lippe) - [`08ae31e`](https://togithub.com/eslint/eslint/commit/08ae31e539e381cd0eabf6393fa5c20f1d59125f) New: Implement cacheStrategy (refs [eslint/rfcs#​63](https://togithub.com/eslint/rfcs/issues/63)) ([#​14119](https://togithub.com/eslint/eslint/issues/14119)) (Manu Chambon) - [`5e51fd2`](https://togithub.com/eslint/eslint/commit/5e51fd28dc773c11c924450d24088f97f2824f00) Update: do not ignore symbolic links (fixes [#​13551](https://togithub.com/eslint/eslint/issues/13551), fixes [#​13615](https://togithub.com/eslint/eslint/issues/13615)) ([#​14126](https://togithub.com/eslint/eslint/issues/14126)) (Pig Fang) - [`87c43a5`](https://togithub.com/eslint/eslint/commit/87c43a5d7ea2018cffd6d9b5c431ecb60caaf0d6) Chore: improve a few comments and fix typos ([#​14125](https://togithub.com/eslint/eslint/issues/14125)) (Tobias Nießen) - [`e19c51e`](https://togithub.com/eslint/eslint/commit/e19c51ea2ef2cf666d94218c66f6b223bb2e9dae) Sponsors: Sync README with website (ESLint Jenkins) - [`b8aea99`](https://togithub.com/eslint/eslint/commit/b8aea9988b6451b6a05af4f3ede8d6ed5c1d9926) Fix: pluralize 'line' to 'lines' in max-lines-per-function description ([#​14115](https://togithub.com/eslint/eslint/issues/14115)) (Trevin Hofmann) - [`f5b53e2`](https://togithub.com/eslint/eslint/commit/f5b53e285ab5286ea382d73b379b820f649c20d0) Sponsors: Sync README with website (ESLint Jenkins) - [`eee1213`](https://togithub.com/eslint/eslint/commit/eee12134ce0956e9f825c1630776b1731551a948) Sponsors: Sync README with website (ESLint Jenkins) - [`5c4d7ea`](https://togithub.com/eslint/eslint/commit/5c4d7ea8d2e8d137c42b918dc99a84b4a5fed9b3) Sponsors: Sync README with website (ESLint Jenkins)

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