Fixed pixi.js version in readme and docs

[TejasBhovad]

Description of change

Because pixi-react doesnt support latest version of pixi.js yet the docs and readme file needed updating to install v7 of pixi.js

Fixes: changed npm install pixi.js @pixi/react to npm install pixi.js@7 @pixi/react

Pre-Merge Checklist

• [x] Documentation is changed or added

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[TejasBhovad]

why do the test cases fail? Also npm install fails

[trezy]

Closing since v7 is no longer the latest version and will only receive security updates moving forward.