deps: Remove webpack from peerDep and move info about ver to readme

[kamilogorek]

After discussing it with the team, we decided to revert this change, as the behavior of peerDependenciesMeta is not consistent across npm/yarn versions.

ref: #343
ref: #350

[belgattitude]

@kamilogorek

the behavior of peerDependenciesMeta is not consistent

Without peerDependencies + Meta (and optional)... I feel it's gonna be a ping-pong between packages managers issues.

Why not betting on the future ?

• From NPM 8 (Oct 07, 2021) and up there was a change about auto-installing peer-deps (see implemented rfc https://github.com/npm/rfcs/blob/main/implemented/0030-no-install-optional-peer-deps.md), the optional is now supported:https://docs.npmjs.com/cli/v8/configuring-npm/package-json#peerdependenciesmeta. In other words they reverted the auto-install from npm 7.
• NPM 8 with --strict-peer-deps will fail without the peer-dep.
• Yarn 1, 2, 3 support optional. https://classic.yarnpkg.com/en/docs/package-json#toc-peerdependenciesmeta
• Pnpm support it too https://pnpm.io/package_json#peerdependenciesmeta

And add a note for npm 6/7 (legacy as per their doc) recommending an update to v8 to avoid downloading webpack twice if not needed ?

Would it be more future-proof ? (Maybe with a major version of the sentry webpack plugin ?)