docs: improve migration from v2 guide

[sapphi-red]

Description

I checked the commit log from 2.9.8 and added descriptions for breaking changes which was missing.

There are potentially breaking changes which I didn't include.

• feat: enable generatedCode: 'es2015' for rollup build #5018
  • Transpile to ES5 is now necessary even if the user code only includes ES5. This is unlikely to happen.
• fix: vite client types #7877
  • /// <reference lib="dom" /> is removed from vite/client.d.ts. This only affects users who does not have either {"lib": ["dom"]} nor {"lib": ["webworker"]} in tsconfig.json.
• chore: use unbuild to bundle plugins #8139
  • exports field now exists. For example, it breaks if someone is importing vite/dist/node/cli.js directly which rarely happens.
• feat: non-blocking esbuild optimization at build time #8280
  • server.force option was removed in favor of force option.

And some which only affects plugin/tool creators which I didn't include.

• refactor: remove deprecated api for 3.0 #5868
  • printHttpServerUrls is removed
  • server.app, server.transformWithEsbuild are removed
  • import.meta.hot.acceptDeps is removed
• chore: do not fixStacktrace #7995
  • ssrLoadModule's fixStacktrace option's default is now false
• feat!: migrate to ESM #8178
  • formatPostcssSourceMap is now async
  • resolvePackageEntry, resolvePackageData are no longer available from CJS build (dynamic import is needed to use in CJS)

Also I didn't include breaking changes of plugins.

• refactor: remove deprecated api for 3.0 #5868
  • options.parserPlugins is removed
• fix(plugin-legacy): respect entryFileNames for polyfill chunks #8247
  • potentially: vite/legacy-polyfills in manifest is now vite/legacy-polyfills-legacy.
  • potentially: polyfills-modern.hash.js is now polyfills.hash.js.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-cat]

Thanks for the PR @sapphi-red 💚

About the ones you list in the description, I think it would be useful to have them in the migration guide in a new Advanced section. Maybe we could directly list the PR as you did here with a line each instead of a detailed explanation. And users can check the complete rationale and context in the PRs. What do you think?

[sapphi-red]

I think it would be useful. I'll create another PR for that! 👍