chore(deps): update dependency imagemin-optipng to v8

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
imagemin-optipng	dependencies	major	^7.1.0 -> ^8.0.0

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Release Notes

imagemin/imagemin-optipng

### [`v8.0.0`](https://togithub.com/imagemin/imagemin-optipng/releases/v8.0.0) [Compare Source](https://togithub.com/imagemin/imagemin-optipng/compare/v7.1.0...v8.0.0) - Require Node.js 10 ([#​26](https://togithub.com/imagemin/imagemin-optipng/issues/26)) [`fd32870`](https://togithub.com/imagemin/imagemin-optipng/commit/fd32870) - Fix missing backquote in the readme ([#​24](https://togithub.com/imagemin/imagemin-optipng/issues/24)) [`552a3ab`](https://togithub.com/imagemin/imagemin-optipng/commit/552a3ab) - Tidelift tasks [`d9b69ac`](https://togithub.com/imagemin/imagemin-optipng/commit/d9b69ac) - Set interlacing to `0` by default, add tests and improve docs [`d439590`](https://togithub.com/imagemin/imagemin-optipng/commit/d439590) - Add `interlaced` option ([#​23](https://togithub.com/imagemin/imagemin-optipng/issues/23)) [`ef42c2d`](https://togithub.com/imagemin/imagemin-optipng/commit/ef42c2d)

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[codecov[bot]]

Codecov Report

Merging #11 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #11   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           13        13           
  Branches         2         2           
=========================================
  Hits            13        13           

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