Update license to latest version

[litetex]

What is it?

• Meta improvement to the project (dev facing)

Description of the changes in your PR

Updates the license to the latest version of https://www.gnu.org/licenses/gpl-3.0.txt
This way also GitHub recognizes the License correctly:
It should look like this after merging this PR:

Due diligence

• I read the contribution guidelines.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[TobiGr]

The license is still not properly recognized:

[litetex]

The license is still not properly recognized: ...

https://stackoverflow.com/questions/66258037/github-license-not-recognised
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/licensing-a-repository#detecting-a-license

root@d9e046c88310:/app# licensee detect .
License:        NOASSERTION
Matched files:  LICENSE, copyright, README.md
LICENSE:
  Content hash:  7d4cdf499d39e2e1ce27b2878e22872f0f5a74dd
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       GPL-3.0
copyright:
  Content hash:  292bc3bb9228726147ae822cb1941bd83a0c3680
  License:       NOASSERTION
  Closest non-matching licenses:
    0BSD similarity:  30.08%
    ISC similarity:   28.19%
    Zlib similarity:  26.23%
README.md:
  Content hash:  8aa4b4336e12b69cec2eee536f7e84c0f83c6f0d
  Confidence:    90.00%
  Matcher:       Licensee::Matchers::Reference
  License:       GPL-3.0
  Closest non-matching licenses:
    WTFPL similarity:  22.92%
    0BSD similarity:   13.02%
    Zlib similarity:   11.30%

Why the hell is there a copyright file?

Create a new PR for fixing this: #8077