build: make crashpad the default backend on Linux

[supervacuus]

We've been discussing this for a year now, and it is a recurring and understandable mental burden for newcomers (as well as developers of integrations within Sentry). From my POV, there is no reason to have breakpad over crashpad as the default backend for Linux besides the worry that we might break builds that rely on that default. Most recent interactions with new users show that people choose crashpad in the end anyway.

I would make this a breaking change toward 0.7.0.

This is also related to the context touched by #908.

I am open to counter-arguments.

[bsergean]

fyi when I tried crashpad, it was requiring some extra permissions which aren't always available in kubernetes (like ability to ptrace a process), which is why we are still using breakpad.

but I understand that's it's a more reasonable default, maybe this constraint should be documented somewhere.