Do not enforce using explicit build types with cmake_multi.

[rasjani]

cmake_multi enforced existence of conanbuildinfo_release.cmake &
conanbuildinfo_debug.cmake had to be present or the build would fail if
conanbuildinfo_multi.cmake was loaded. This is now changed so that
only generated conanbuildinfo files are loaded.

Fixes #7253

Changelog: Fix: Avoid requiring the existence of all conanbuildinfo_xxx.cmake files in cmake_multi generator.
Docs: Omit

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[jgsogo]

LGTM

[memsharded]

This had a reason at the moment, mostly to protect users that did install other configurations later, after cmake project generation, that weren't being automatically loaded without regenerating. I agree this was mostly an anti-feature, better to be more flexible and allow users to do what they want without unnecessarily protecting them.

Thanks very much for contributing this.