Deprecate non boolean if syntax

[proux01]

Deprecate the non boolean if t then _ else _ syntax (in favour of if t is <first_constructor> then _ else _).

Depends on: #21609

• Added / updated test-suite.

• Added changelog.
• Added / updated documentation.
  • Documented any new / changed user messages.
  • Updated documented syntax by running make doc_gram_rsts.

[SkySkimmer]

Is printing of non booleans changed to use the new syntax too?

[proux01]

I'm working on it (in a separate PR)