Update GPL license file from v2->v3

[antonio-leblanc]

Updates the main LICENSE file from the GNU General Public License version 2 to version 3.
Reasons for considering the update:

• GPLv3 offers improved patent protection clauses.
• Includes anti-Tivoization provisions.
• Provides clearer language and better internationalization.
• Addresses ambiguities found in GPLv2.
• Maintains the strong copyleft nature consistent with the previous GPLv2 license.

The source file headers have already been updated to refer generally to the LICENSE file

[antonio-leblanc]

will create again because it ran a check on master wrongly