Drop footer line warning for packages requiring Emacs 30.1

Drop footer line warning for packages requiring Emacs 30.1

The reason for warning about a missing footer line (";;; foo.el ends
here") is that package.el up until version 27.1 would refuse to
install a package without it.  Emacs 27.1 or later will install such
packages, but will issue a warning, the purpose of which is to
encourage package authors not to break backwards-compatibility.

However, if the minimum required Emacs version for a package is 30.1,
we do not need to worry about compatibility with earlier versions of
Emacs -- the package author has already explicitly said that the
package will not work on earlier versions.  For such packages, there
is no need to warn about a missing footer line.

In the future, this warning could be removed, but it is premature to
do that now.  (See Bug#26490.)  Thus, for packages that does not
specify a minimum version of Emacs, we continue to issue the warning.
We will also continue to warn for packages requiring Emacs 27 to 29,
since those versions will themselves warn if the footer is missing.

* lisp/emacs-lisp/package.el (package-buffer-info): Don't warn if the
footer line is missing for packages requiring Emacs 30.1 or later.