core: change 'found the format marker' log message from info to debug

[mgi388]

Fixes #272

As in #272, I've also experienced the info log flooding for a message that shouldn't really be info. I know we can look at configuring log levels, but the content of the message seems very much debug-like, not informational.

...
2024-05-10T00:15:25.274616Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
2024-05-10T00:15:25.276794Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
2024-05-10T00:15:25.286236Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
2024-05-10T00:15:25.300764Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
2024-05-10T00:15:25.302995Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
2024-05-10T00:15:25.312544Z  INFO symphonia_core::probe: found the format marker [52, 49, 46, 46] @ 0+2 bytes.
...

Relates to #228.

[mgi388]

@pdeljanov unlike #228, this one should hopefully be an easy and uncontroversial merge 🙏