Bump github.com/go-sql-driver/mysql

[mpolden]

While trying to understand why my project pulled in
google.golang.org/appengine/cloudsql indirectly, I noticed that it's pulled in
through this library's dependency on github.com/go-sql-driver/mysql.

$ go mod why -m google.golang.org/appengine
# google.golang.org/appengine
github.com/mpolden/zdns/sql
github.com/jmoiron/sqlx
github.com/jmoiron/sqlx.test
github.com/go-sql-driver/mysql
google.golang.org/appengine/cloudsql

However, I then noticed that a newer version of the dependency removes
google.golang.org/appengine/cloudsql as it's obsolete: go-sql-driver/mysql#1007.

[coveralls]

Pull Request Test Coverage Report for Build 133

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.751%

---

Totals
Change from base Build 122:	0.0%
Covered Lines:	1000
Relevant Lines:	1476

---

💛 - Coveralls

[jmoiron]

Thanks, sorry I missed this, done via another PR. 🙇