Bump version to 26

[jamesmunns]

This should have been done for #497

CC #495, which will bump to v27

[jamesmunns]

I need to come back and cover PmbusStatusError (and SpError::PmbusStatus), PmbusStatusReadError is covered.

Edit: done!

[jamesmunns]

If you'd like, I can back this change out, but I think it's useful. It means we get errors like this:

thread 'versioning::v26::get_pmbus_status_error' panicked at gateway-messages/tests/versioning/v26.rs:121:9:
assertion `left == right` failed: incorrect serialization of PmbusStatus(UnknownRail)
  left: [123, 0]
 right: [39, 0]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

instead of:

thread 'versioning::v26::get_pmbus_status_error' panicked at gateway-messages/tests/versioning/mod.rs:48:5:
assertion `left == right` failed: incorrect serialization of PmbusStatus(UnknownRail)
  left: [123, 0]
 right: [39, 0]
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace