Adds the Discord slash command to get the schedule for the next few
weeks.
Also updates the required Go version to 1.21 to benefit from the new
`time.DateOnly` format that's gonna be used in the absences table.
* Add migration scripts
* Add queries to add, get and list absences
* Add tests for the above
* Add function to get the date of the next Monday
Also fixes potential bug in the build where the output binary would be
written to a file called `./bin` rather than a file named after the
source directory in a _directory_ called `./bin`.
Related to #17
Renames the `/ping` command to `/info` and adds the following
information to the Discord response
* Uptime of the server
* Total number of claims currently held
* Total number of unique players holding claims
William Perron