Remove deprecated rules

[ybiquitous]

As of v15.0.0, we have deprecated many stylistic rules. And, as we announced when the v15.0.0 release, we will remove these rules in the next major version (maybe v16.0.0).

Additionally, as #6753 introduced, the Stylelint community has provided 3rd-party plugins as an alternative, e.g.

• https://github.com/elirasza/stylelint-stylistic

So, I think we can perform this removal without a big confusion.

See also:

• stylelint/docs/migration-guide/to-15.md

  Lines 12 to 14 in e56fc6c

  	### Deprecated stylistic rules
  	We've deprecated [76 of the rules that enforce stylistic conventions](../user-guide/rules.md#deprecated), e.g. `indentation`.

• stylelint/docs/user-guide/rules.md

  Lines 334 to 336 in e56fc6c

  	## Deprecated
  	These rules are deprecated — we won't fix bugs nor add options, and we will remove them in the next major release. We recommend you use a pretty printer (like Prettier) alongside Stylelint rather than these rules.

[jeddy3]

Done in #6979

[aiddroid]

If you deperacated the indentation rule, Can you tell the user what the alternatives are? Very frustrating.

[BeardedFish]

If you deperacated the indentation rule, Can you tell the user what the alternatives are? Very frustrating.

You should be able to use the indentation rule from https://github.com/stylelint-stylistic/stylelint-stylistic via the @stylistic/indentation rule.