Multiple improper SPDX tags

[lemenkov]

Abstract

Lots of source files contain SPDX-License-Identifier: GPL-3.0 while stating in a human-readable license header that

        solidity is free software: you can redistribute it and/or modify
        it under the terms of the GNU General Public License as published by
        the Free Software Foundation, either version 3 of the License, or
        (at your option) any later version.

A correct SPDX license tag for these files has to be GPL-3.0-or-later.

Pull request

I don't think I am in a position to propose PR with changes solely related to a project's licensing terms. Perhaps active Solidity developers should handle it instead.