Everywhere: Use "the SerenityOS developers." in copyright headers

We had some inconsistencies before:

- Sometimes "The", sometimes "the"
- Sometimes trailing ".", sometimes no trailing "."

I picked the most common one (lowecase "the", trailing ".") and applied
it to all copyright headers.

By using the exact same string everywhere we can ensure nothing gets
missed during a global search (and replace), and that these
inconsistencies are not spread any further (as copyright headers are
commonly copied to new files).

Kernel/Net/NE2000NetworkAdapter.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, the SerenityOS developers
+ * Copyright (c) 2021, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */