chore(plugin-pwa): change core-js version in package.json to v3

[Josh-Cena]

Motivation

We already use core-js v3 in the actual code (in babel settings), but it's funny that we still declared core-js v2 in package.json. This hasn't caused much trouble so far because the core provides core-js v3.

We can't rid ourselves of core-js v2 entirely, however, because react-live@2.2.3 depends on it. react-live doesn't work with React v17 yet (#5556), and it's on my hate-list of dependencies😅 Users who use pwa but not live-codeblock may be able to get rid of core-js in their node_modules.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

The PWA still works in deploy preview

[netlify]

✔️ [V2]

🔨 Explore the source changes: 4e4d4a9

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/619cd7665da1ef000965434f

😎 Browse the preview: https://deploy-preview-5995--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	77
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5995--docusaurus-2.netlify.app/

[github-actions]

Size Change: -1 B (0%)

Total Size: 892 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.3 kB	0 B
website/build/assets/css/styles.********.css	100 kB	0 B
website/build/assets/js/main.********.js	477 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.2 kB	0 B
website/build/blog/index.html	37.8 kB	0 B
website/build/docs/index.html	44 kB	+1 B (0%)
website/build/docs/installation/index.html	51.7 kB	-2 B (0%)
website/build/index.html	29.5 kB	0 B
website/build/tests/docs/index.html	25.2 kB	0 B
website/build/tests/docs/standalone/index.html	21.7 kB	0 B

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