Migrate to ESLint major version 8 (#93)

Before the supported ESLint major version for all npm packages of this
styleguide was still `7` which has now been updated to `8`,
released on 2021-10-09 [1].
To update all packages, the official ESLint 8 migration guide [2] was
used to adapt the breaking changes for users [3], but none of them
affected this project so there were no further steps required other than
updating the corresponding package (`dev`/`peer`)`dependencies`.

The following plugins have been updated to their latest major & minor
versions to match the supported ESLint version:

- `eslint` [14] - `^7.32.0` → major version `8` [1]
  The currently latest version is `^8.39.0` [4] which is now used as
  minimal requirement.
- `eslint-plugin-prettier` [5] - `^3.4.1` → major version `4` [6]
  The currently latest version is `4.2.1` [7] which is now used as
  minimal requirement.
  - Dropped support for older versions of ESLint, Prettier and Node.
    It now requires at least ESLint `>=7.28.0 <8.0.0`,
    Prettier `>=2.0.0 <3.0.0` and Node `>=12.0.0`.
- `eslint-find-rules` [8] - `3.6.1` → major version `4` [9]
  The currently latest version is `4.1.0` [10] which is now used as
  minimal requirement.
  - Comes with support for ESLint 8.
- `@typescript-eslint/eslint-plugin` [11] - `^4.33.0` → major
  version `5` [13]
  The currently latest version is `5.59.1` [10] which is now used as
  minimal requirement.
  - Comes with support for ESLint 8.
  - Dropped support for Node `<=10`.
  - Dropped support for ESLint `<=5`.
- `@typescript-eslint/parser` [15] - `^4.33.0` → major version `5` [13]
  The currently latest version is `5.59.1` [10] which is now used as
  minimal requirement.
  - Comes with support for ESLint 8.
  - Dropped support for Node `<=10`.
  - Dropped support for ESLint `<=5`.
- `typescript` [16] - `^4.9.5` → major version `5` [17]
  The currently latest version is `5.0.4` [18] which is now used as
  minimal requirement.
  - Dropped support for Node `<=12`.

[1]: https://github.com/eslint/eslint/releases/tag/v8.0.0
[2]: https://eslint.org/docs/latest/use/migrate-to-8.0.0
[3]: https://eslint.org/docs/latest/use/migrate-to-8.0.0#breaking-changes-for-users
[4]: https://github.com/eslint/eslint/releases/tag/v8.39.0
[5]: https://github.com/prettier/eslint-plugin-prettier
[6]: https://github.com/prettier/eslint-plugin-prettier/blob/master/CHANGELOG.md#v400-2021-08-30
[7]: https://github.com/prettier/eslint-plugin-prettier/releases/tag/v4.2.1
[8]: https://github.com/sarbbottam/eslint-find-rules
[9]: https://github.com/sarbbottam/eslint-find-rules/releases/tag/v4.0.0
[10]: https://github.com/sarbbottam/eslint-find-rules/releases/tag/v4.1.0
[11]: https://github.com/typescript-eslint/typescript-eslint/tree/main/packages/eslint-plugin
[12]: https://github.com/typescript-eslint/typescript-eslint/releases/tag/v5.59.1
[13]: https://github.com/typescript-eslint/typescript-eslint/releases/tag/v5.0.0
[14]: https://github.com/eslint/eslint
[15]: https://github.com/typescript-eslint/typescript-eslint/tree/main/packages/parser
[16]: https://github.com/Microsoft/TypeScript
[17]: https://devblogs.microsoft.com/typescript/announcing-typescript-5-0
[18]: https://github.com/microsoft/TypeScript/releases/tag/v5.0.4

GH-92