[BOLT] Switch default perf2bolt format to YAML

YAML embeds metadata for stale matching, making it better suited to cases with non-zero gap (profiled != pre-bolt binary). YAML profile is at parity with fdata in terms of optimizations.

Since `-o` output format is controlled by `output-format`, the switch can be straightforward. `-w` flag can be kept for backwards compatibility with a warning (still producing yaml, redirecting users to `output-format`), but eventually dropped.

Locate perf2bolt's handling of the output-format setting and the legacy -w option; start by tracing how -o selects the output format. Done means YAML is the default, -w remains supported with a warning and redirects users to output-format, and existing behavior is covered by the relevant tests.

Written by the indexing model from the issue text.