[T8code] update to version 4

[benegee]

Update to newly released t8code version 4.0.0

• p4est patch is not required anymore
• enable Fortran interface
• macos builds fail due to std::visit being unavailable (introduced in macOS 10.14) (https://buildkite.com/julialang/yggdrasil/builds/25797/steps/canvas?sid=019b07c5-c4d9-4378-b026-c387ade12c8e#019b07c5-c535-46ce-bf31-3566ea7cbce7/1104-4985) using gcc as suggested here https://docs.binarybuilder.org/stable/build_tips/#Using-GCC-on-macOS-and-FreeBSD works!
• windows builds fail because MPI 3 is required, but only 2 is available (https://buildkite.com/julialang/yggdrasil/builds/25797/steps/canvas?sid=019b07c5-c4e2-4a5b-8145-bc946d77d76d#019b07c5-c544-42bb-bf37-d4ef16b51cc0/1096-2651) I have no idea here, so far the time being I disabled builds on Windows
• same for riscv64 (https://buildkite.com/julialang/yggdrasil/builds/25797/steps/canvas?sid=019b07c5-c4e0-423f-bdb4-77c1f1b6ed6c#019b07c5-c541-4ab3-92fa-715e6be9f0df/1092-2647)

Is this acceptable, @spenke91, @JoshuaLampert, anyone else?