db schema: Use `hw_m2_slot` enum instead of ints for `inv_sled_config_reconciler`

[jgallagher]

The inv_sled_config_reconciler table uses an INT2 column with a check constraint for storing the boot disk slot:

omicron/schema/crdb/dbinit.sql

Lines 3721 to 3727 in 0bef6e9

	-- Which internal disk slot did we use at boot?
	--
	-- If not NULL, `boot_disk_slot` must be 0 or 1 (corresponding to M2Slot::A
	-- and M2Slot::B, respectively). The column pair `boot_disk_slot` /
	-- `boot_disk_error` represents a Rust `Result`; one or the other must be
	-- non-NULL, but not both.
	boot_disk_slot INT2 CHECK (boot_disk_slot >= 0 AND boot_disk_slot <= 1),

#8624 adds a hw_m2_slot enum; we should use that instead. I think this will require the normal dance of

• rename old column
• add new column
• copy data from old to new
• drop old column

and we may also want to do the same to the boot_disk_error column just to keep them next to each other in the schema? (That may be overkill; certainly not necessary for crdb but might be nicer for readers.)