Upgrade to bootloader 0.11

[gjf2a]

Currently uses bootloader 0.9.8. Does not compile with bootloader 0.11.

[gjf2a]

This would require updating output to be pixel-based instead of VGA-based. I don't want to do that yet - it would be a lot of work. But I am leaving this open in case I change my mind. See https://github.com/rust-osdev/bootloader/blob/086c248e89ec3519f77dadda6c74d193e6fa22fd/docs/migration/v0.9.md.

[gjf2a]

This absolutely needs to happen now - the old bootloader JSON file breaks with the current nightly.