linux-postmarketos-qcom-msm8996: Change repository location and upgrade to 5.7

Updates linux-postmarketos-qcom-msm8996 to 5.7 and switches to a shared repository. This fixes #559 (closed) but does not solve panel selection for the 3T.

Side note, I not going to able to maintain this or my other WIP Qualcomm SoC device ports for the next few months due to some personal issues, so if anyone is interested in taking up development/maintainership for this repository, please send a request to be added to the msm8996-mainline GitLab group.