Simplify and extend /releases/ redirects from legacy v2.ocaml.org URLs

[cuihtlauac]

• Turn v2 urls from data/releases/*.md file into w/o v2. urls
• Remove manual /releases/ redirects from redirection.ml
• Handle /releases/ redirects in middleware.ml

This fixes some broken links in release notes and manuals by supporting all the /releases/* urls as in v2 and simplifies release process.

[cuihtlauac]

@shakthimaan: any views on this?

[shakthimaan]

LGTM!