The hope is that this will be clearer and less annoying for users.

A user came to us a couple weeks ago stating that it was confusing
that the `brew upgrade` command printed an error when a pinned
formula had a new version available and didn't get upgraded.

This PR changes that message to a warning from an error. While looking
into this we found that there is another message that gets printed
when a package dependency doesn't get upgraded because it is pinned
and that got turned into a warning from a normal message. Honestly,
that should be more worrying for the user anyway; it could lead to
a program not working correctly in the worst case.

I also added a message to the `brew pin` command warning about
potential unintended behavior if a dependency gets pinned and another
package requires a newer version of it.

Lastly, I added a commented out deprecation notice for the
`brew upgrade --ignore-pinned` command since it's now the default.

The idea here is that it should be expected that `brew upgrade` will
not upgrade pinned packages but will attempt to upgrade everything else.
For that reason, it will only show a warning about pinned packages
in that case.

If, on the other hand, you pass the name of a pinned package explicitly
to the upgrade command, like in `brew upgrade PINNED`, we want to show
an error since we cannot upgrade that package until it gets unpinned.