Remove deprecated hosts table columns (gigs_disk_space_available and percent_disk_space_available)

[mna]

Goal

With the work done on #7592 , the host's disk space information has been moved from the hosts table to the host_disks table, and a lazy migration approach was used to make migrating fast (i.e. the information from the hosts table was not copied to the new table, instead we waited for the hosts to report their disk space data again, and inserted it in the new table at that moment - see #7592 (comment)).

In a subsequent release, we should add a migration to remove the now-deprecated gigs_disk_space_available and percent_disk_space_available fields in the hosts table.

How?

Frontend

No impact.

Backend

Implement a migration to remove those two columns. Test with many hosts in the table, to make sure the migration is still reasonably fast.

If os_versions is no longer being used, please remove it. If it is still being used and will take more than an hour to fix, let's address it later.

[mna]

@lukeheath as a follow-up to #7592 , we should plan this in a not-too-distance future release, to prevent keeping obsolete columns around.

[lukeheath]

@mna Great point, thanks for creating this card. I've brought it into the "Specified" column so we can estimate in and get it prioritized.

[mna]

If os_versions is no longer being used, please remove it. If it is still being used and will take more than an hour to fix, let's address it later.

The hosts.os_version column is still widely used in host and label queries, from a quick search. Seems like it should be a ticket by itself to remove its use and drop the column.

[mna]

Quite fast with 40K+ hosts on my laptop.

mysql> select count(*) from hosts;
+----------+
| count(*) |
+----------+
|    42209 |
+----------+
1 row in set (0.00 sec)

$ time ./build/fleet prepare db --dev
2022/11/09 14:51:26 [2022-11-09] Remove Hosts Disk Space Columns
Migrations completed.

real	0m2.062s
user	0m0.062s
sys	0m0.030s