api: revert breaking change in proto

Commit b6d3583 introduced a breaking change in our
protobuf API by renaming the existing stack_trace field in the kprobe message to
kernel_stack_trace when introducing user stack traces. While a change of this nature is
not ordinarily problematic in protocol buffer APIs, it is an issue for Tetragon
specifically since we rely on protojson for event exports. This effectively means that
renaming a field breaks compatibility for downstream consumers of event exports. Revert
the rename to prevent breakages.

Signed-off-by: William Findlay <[email protected]>