Old toolchain is now deprecated

[inclement]

Buildozer now uses the current p4a toolchain automatically, and I haven't even seen anyone discussing the old toolchain for some time. As such, and given that nobody has contributed anything to it for a long time, the next release will officially fully deprecate the old toolchain.

I've opened this issue as a reminder, both for the issue notes, and because as part of this we should:

• Close all issues and PRs relating only to the old toolchain
• Make sure the documentation is updated if necessary