fix: further dependency export cleanup

[supervacuus]

• Use THREADS_PREFER_PTHREAD_FLAG as is recommended.
• Replace checks for _FOUND variables with checks to actual targets before we find_package() in our build scripts.
• Ensure that we include the crashpad targets in the export configuration only after all invocations of find_dependency()
• Remove REQUIRED from the find_dependency() invocations and let it retrieve the value from the find_package() invocations in the build scripts.
• link _SENTRY_PLATFORM_LIBS only as PRIVATE to target sentry since it sufficiently exports in case of a static build to the consuming project (using LINK_ONLY generator expressions in exported targets).
• corresponding changes in crashpad.

• Update submodule reference after the merge in our crashpad fork.