Stop Pharo 11 support

[LabordePierre]

Related to https://github.com/OpenSmock/Pyramid/pull/280/changes#r4080233759 we stop Pharo 11 support.
Task list:

• Create a branche named "Pharo11" from the last Pharo 11 supported commit
• Remove the Pharo 11 tag from the readme file
• Create a release which act this point

[LabordePierre]

Last release which support Pharo 11: https://github.com/OpenSmock/Pyramid/releases/tag/alpha12

[LabordePierre]

Branch here: https://github.com/OpenSmock/Pyramid/tree/Pharo11