Implement #5971. Free text fields preserve indentation

[phadej]

Starting with cabal-version: 3.0 free text fields
preserve indentation and blank lines.
In other words, we don't need single dots to indicate blank line.

IPI is not versioned. So we default to latest variant always.
This is little harm, as old GHC use old Cabal to print
IPIs. And free text fields are only informative.

Fixes #5938

---

category:            Data, Testing
description:
  A line before

  * foo

      * bar

      * quux

  @
  foo = bar
    where
      bar = foo

  quux = 42
  @

generates

---

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

[hvr]

LGTM