Feature: add deprecated attribute

[SSE4]

closes: #7251

Changelog: Feature: add deprecated attribute
Docs: conan-io/docs#1775

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[SSE4]

NOTE: this doesn't print:

WARN.- Recipe 'cpp-taskflow/1.0' is deprecated in favor of 'taskflow/1.0'. Please, consider changing your requirements.

instead, it will print just:

WARN.- Recipe 'cpp-taskflow/1.0' is deprecated in favor of 'taskflow'. Please, consider changing your requirements.

notice taskflow has no version specified.
reason - taskflow/1.0 might not exist - the new version might be taskflow/2.0 for instance.

[danimtb]

I was wondering if this deprecated attribute is prone to be ser conditionally in the configure() method. Having a look at the implementation, I think it is already working but I am not sure if we have a use case for that 🤔

[memsharded]

I was wondering if this deprecated attribute is prone to be ser conditionally in the configure() method. Having a look at the implementation, I think it is already working but I am not sure if we have a use case for that 🤔

Well, it is possible that some recipe support for one OS is deprecated, probably not a very common case, so I would be fine even if this didn't work at the moment.

[SSE4]

I don't see any advantage in such complicated use-case as a conditionally deprecated recipe. I'd say let's go with simple implementation, until someone will actually request that feature.

[uilianries]

One advantage using the current condition, is that we can give a hint to the new reference. This new feature doesn't offer the same case.

Maybe, deprecated could accept a package reference or a boolean:

class Foo(ConanFile):
    ...
    deprecated = True
    # OR
    deprecated = "foobar/0.1.0" # new reference

[SSE4]

@uilianries both are allowed, boolean and reference

[uilianries]

@memsharded Sorry, now I see: https://github.com/conan-io/conan/pull/7399/files#diff-3eb9b6f2733f2b3ba7899d8edeebf60fR42