[13.x] Align Mailable::cc @return with sibling fluent methods

[mosabbirrakib]

In Illuminate\Contracts\Mail\Mailable, every fluent setter documents @return \$this except cc(), which is the lone outlier with @return self:

public function cc($address, $name = null);   // @return self
public function bcc($address, $name = null);  // @return $this
public function to($address, $name = null);   // @return $this
public function locale($locale);              // @return $this
public function mailer($mailer);              // @return $this

The actual implementation Illuminate\Mail\Mailable::cc() already documents @return \$this and returns \$this via setAddress(). Updates the contract to match, so static analysers see the more precise type for \$mailable->cc(...)->bcc(...) chains on a typed contract reference.

PHPDoc-only change; no behavioral or runtime impact.