A deprecated enum member only had its runtime message, so every one
needed a hand-written `Deprecated:` notice repeating the same
information, even for a plain rename. Client libraries can potentially
deprecate (rename) dozens of members in a release.

`deprecated_member()` and `DeprecatedMember` now take the same
structured information as `DeprecatedAlias`: a keyword-only `new_name`,
the member to use instead in the same enum, and `since`, the version it
is deprecated since, which is only documentation metadata for
`griffe-frequenz-core`. `message` gets a `None` default, and at least
one of `message` and `new_name` is required, which overloads let mypy
check.

    PENDING = deprecated_member(1, new_name="OPEN", since="v1.2.0")

Without `message`, the warning is generated as `<Enum>.<NAME> is
deprecated. Use <Enum>.<new_name> instead.`, with fully qualified names,
as the deprecations guide asks. It can only be built when the enum class
is created, since only then are its module and qualified name known, so
the metaclass generates it then. It also checks that `new_name` is a
member of the same enum and is not deprecated itself, so a typo or a
chain of deprecations fails at import time instead of pointing users at
something that doesn't work.

`message` stays a literal message rather than an `{old}`/`{new}`
template like `DeprecatedAlias`'s, since existing messages may contain
braces, and it is still not checked, as before.

Existing calls keep working, since `value` and `message` keep their
position and names and the new parameters are keyword-only. The only
visible change is that `DeprecatedMember.message` is now `str | None`,
as a generated message is not known when the member is declared. The
class is documented as not meant to be used directly, so this is
acceptable in a minor release; storing the generated message back into
it would have kept the type but left it without a meaningful value
until the class is created.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

The `Enum`, `unique()` and README examples still wrote messages like
`PENDING is deprecated, use OPEN instead`, which the deprecations guide
no longer recommends: a warning has to start with the fully qualified
name, which the own-deprecations `pytest` filter relies on, and the
version belongs in the documentation notice.

They now use the structured `deprecated_member()` form for renames, and
the `Enum` example also shows a member with no replacement, which gives
its own fully qualified `message` and needs a hand-written notice. The
text after it says what each one warns and how it is documented.

The `Enum` example also claimed that `TaskStatus["DONE"]` and
`TaskStatus(3)` were `TaskStatus.FINISHED`, which had a different value,
and went unnoticed because the examples are only linted, not run. The
new example asserts what actually happens.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>