chore: rewrite HTML tables in migrating-4.mdx to markdown

[krzysdz]

Tables in migrating-4.mdx are simple an have been rewritten using markdown syntax, which makes them work better with plugins (the HTML was missing <thead> and <tbody>).

The tables in "Express behind proxies" are quite large and contain a lot embedded markdown (code blocks, lists). Unfortunately, as far as I know it is not possible to write a markdown table with lists or code blocks without using HTML for either the table or the content.

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[krzysdz]

One thing that I noticed while making these changes: #2221 removed a table from 5.x writing middleware docs, but left it in 4.x. The table served as an explanation of some things shown on an illustration, but the illustration has been updated to include these descriptions, so 4.x version is labelled twice.

[ShubhamOulkar]

#2493 fixes scrolling issues, also we discussed on rewriting to markdown tables. migrating-4 tables are in markdown, if you want then do it for tables on behind-proxies page. Removing manual addition of table-scroller is essential before merging the PR.

[krzysdz]

Removed the manually added .table-scroller. Tables from behind-proxies.mdx cannot be rewritten to pure markdown.

I looked for inline HTML and other than this (cannot be removed), some other HTML that can't be removed (e.g. links wrapping Astro components) it can be found only in:

• 3.x docs
• an unordered list in behind-proxies (4.x and 5.x, see krzysdz/expressjs.com@0afd270)
• one blog post (see Fix rehype accessible table plugins #2493 (comment))