Update autoprefixer requirement from ^8.5.1 to ^9.1.5

[dependabot-preview[bot]]

Updates the requirements on autoprefixer to permit the latest version.

Release notes

*Sourced from [autoprefixer's releases](https://github.com/postcss/autoprefixer/releases).* > ## 9.1.5 > * Remove `@babel/register` from dependencies.

Changelog

*Sourced from [autoprefixer's changelog](https://github.com/postcss/autoprefixer/blob/master/CHANGELOG.md).* > ## 9.1.5 > * Remove `@babel/register` from dependencies. > > ## 9.1.4 > * Use Babel 7. > > ## 9.1.3 > * Sort properties in `autoprefixer --info` alphabetically. > * Fix old Firefox gradient prefix. > > ## 9.1.2 > * Fix `autoprefixer --info` in new Node.js. > > ## 9.1.1 > * Retain `grid-gap` through `[**media**](https://github.com/media)` (by Bogdan Dolin). > * Fix `grid-template` and `[**media**](https://github.com/media)` (by Bogdan Dolin). > * Fix Grid areas searching error (by Bogdan Dolin). > * Fix `span X` Grid prefix (by Bogdan Dolin). > * Fix docs (by Eduard Kyvenko). > > ## 9.1 “Equality before the law” > * Add `background-clip: text` support. > * Fix adding Grid span for IE (by Bogdan Dolin). > > ## 9.0.2 > * Show warning on Grid area names conflict (by Bogdan Dolin). > * Fix documentation (by Sven Wagner). > > ## 9.0.1 > * Fix nested at-rules in Grid prefixes (by Ivan Malov). > > ## 9.0 “A Mari Usque Ad Mare” > * Remove Node.js 9 and Node.js 4 support. > * Remove IE and “dead” browsers from Babel. > * Use PostCSS 7.0. > * Use Browserslist 4.0. > > ## 8.6.5 > * Do not show Grid warnings if IE was not selected. > > ## 8.6.4 > * Fix `stretch` prefix in Chrome >= 46. > > ## 8.6.3 > * Add warnings for unsupported Grid features. > * Add warnings about wrong Grid properties. > * Add note about `grid` option for grid properties in `autoprefixer --info`. > > ## 8.6.2 > * Fix error during adding Grid prefixes in `[**media**](https://github.com/media)` (by Evgeny Petukhov). > ... (truncated)

Commits

- See full diff in [compare view](https://github.com/postcss/autoprefixer/commits/9.1.5)

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[dependabot-preview[bot]]

Superseded by #28.