Migrating to 15.0.0 guide.

• Removed: Node.js 12 support (#6477) (@ybiquitous). (BREAKING)
• Removed: support for processors (#6479) (@ybiquitous). (BREAKING)
• Removed: syntax option (#6420) (@fpetrakov). (BREAKING)
• Changed: extends in overrides to merge to be consistent with plugins behaviour (#6380) (@jasikpark). (BREAKING)
• Changed: type definitions to reorganize (#6510) (@ybiquitous). (BREAKING)
• Changed: type names to be more consistent (#6503) (@ybiquitous). (BREAKING)
• Deprecated: stylistic rules handled by Prettier (#6504) (@ybiquitous).
• Added: declaration-property-value-no-unknown rule (#6511) (@jeddy3).
• Added: media-feature-name-unit-allowed-list rule (#6550) (@mattxwang).
• Added: function-url-quotes autofix (#6558) (@mattxwang).
• Added: ignore: ["custom-elements"] to selector-max-type (#6588) (@muddv).
• Added: ignoreFunctions: [] to unit-disallowed-list (#6592) (@mattxwang).
• Added: deprecated rule warnings (#6561) (@ybiquitous).
• Added: message arguments to declaration-property-unit-allowed-list (#6570) (@mattxwang).
• Fixed: overrides.files in config to allow basename glob patterns (#6547) (@ybiquitous).
• Fixed: at-rule-no-unknown false positives for @scroll-timeline (#6554) (@mattxwang).
• Fixed: function-no-unknown false positives for interpolation and backticks in CSS-in-JS (#6565) (@hudochenkov).
• Fixed: keyframe-selector-notation false positives for named timeline ranges (#6605) (@kimulaco).
• Fixed: property-no-unknown false negatives for newer custom syntaxes (#6553) (@43081j).
• Fixed: selector-attribute-quotes false positives for "never" (#6571) (@mattxwang).
• Fixed: selector-not-notation autofix for "simple" option (#6608) (@Mouvedia).