Remove separate 2.x branch for older Magento versions

[hnsr]

Having to keep two branches up to date with install.sh en README.md changes is a bit of a hassle (or more importantly: we just forget to keep them updated). Since (I think?) the only difference between the branches is having to apply a different static content patch (depending on M2 version), I propose we just handle this by having only a main branch with the two patches in the repo with appropriate instructions in a single unified README

[PascalBrouwers]

Seems like that is the only change indeed (other than 2.x running php 7.3 by default)

[hnsr]

Removed the 2.x branch and references in the README. Existing Magento 2.3.x projects should still work