Remove deprecated syntax option

[ybiquitous]

When releasing v14.0.0, we deprecated the syntax option to require using the customSyntax option instead.

stylelint/docs/migration-guide/to-14.md

Line 17 in d402889

### `syntax` option and automatic inferral of syntax

With the next major version, we can remove the deprecated option. See below:

stylelint/types/stylelint/index.d.ts

Lines 243 to 244 in d402889

	/** @deprecated Use `customSyntax` instead. Using this option will result in an error. */
	syntax?: string;

[fpetrakov]

@ybiquitous I guess we can close this issue?

[ybiquitous]

@fpetrakov Ah, thanks. I forgot that auto-closing doesn't work on the non-main branches. 😅