fix: move docusaurus core back to hard dependencies

[pranabdas]

While upgrading to latest beta 10, it gives @docusaurus/core peer dependency error due to different versions required by @docusaurus/preset-classic and root @docusaurus/core. Current canary release seems broken as well because of this inconsistent requirements.

Motivation

Trying to fix the peer dependency error while upgrading to the latest beta 10.

Details of error:

$ npm i @docusaurus/core@latest @docusaurus/preset-classic@latest
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR! 
npm ERR! While resolving: XXXXXXXX@0.2.1
npm ERR! Found: @docusaurus/core@2.0.0-beta.10
npm ERR! node_modules/@docusaurus/core
npm ERR!   @docusaurus/core@"2.0.0-beta.10" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peer @docusaurus/core@"2.0.0-beta.9" from @docusaurus/preset-classic@2.0.0-beta.10
npm ERR! node_modules/@docusaurus/preset-classic
npm ERR!   @docusaurus/preset-classic@"2.0.0-beta.10" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
npm ERR! 
npm ERR! See /Users/XXXX/.npm/eresolve-report.txt for a full report.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/XXXX/.npm/_logs/2021-12-XXXX_30_21_115Z-debug.log

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Sorry, I don't know how to test npm releases.

[netlify]

✔️ [V2]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	76
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6078--docusaurus-2.netlify.app/

[armano2]

maybe we should make it an range? ^2.0.0

[ericvicenti]

I don't think ranges work for beta tags. But I could be wrong

[Josh-Cena]

Caret ranges do work: https://docs.npmjs.com/cli/v6/using-npm/semver#prerelease-tags

I will probably be going for a range, because if lerna doesn't want to update peer dependencies (#6047 (comment)) we'd have to make it future-proof

[slorber]

For now I'm reverting the peerDep to a regular dep (how it used to be)