Changed license to GPL-2.0+ on 3.2

[krashish8]

Changes proposed in this pull request:

• Changed license to GPL-2.0+
• Ports Change license to GPL-2.0+ on 3.1 #1853 to 3.2

@pgRouting/admins

[dkastl]

This got already merged, but I think the correct term is not using the +, so it shouldn't be named GPL-2.0+, because that's in my opinion deprecated, see my comment here and the URL's #1851 (comment)

So correct would be GNU General Public License v2.0 or later or short GPL-2.0-or-later and it would be possible to link in the Readme to https://spdx.org/licenses/GPL-2.0-or-later.html
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