add Go 1.16 to the build matrix

[shogo82148]

No description provided.

[shogo82148]

SQL2017 is already covered at

go-mssqldb/appveyor.yml

Line 25 in 045585d

SQLINSTANCE: SQL2017

so I dropped this case.

[robinknaapen]

From what I can see every matrix entry needs a SQLINSTANCE since there's no global setting 🤔

[shogo82148]

The build fails due to #639
https://ci.appveyor.com/project/denisenkom/go-mssqldb/builds/38567703/job/mpadty14o33sx2re

panic: Not implemented [recovered]
	panic: Not implemented
goroutine 2183 [running]:
testing.tRunner.func1.2(0x64b360, 0x742b20)
	c:/go116/src/testing/testing.go:1144 +0x49f
testing.tRunner.func1(0xc000088480)
	c:/go116/src/testing/testing.go:1147 +0x695
panic(0x64b360, 0x742b20)
	c:/go116/src/runtime/panic.go:971 +0x49a
github.com/denisenkom/go-mssqldb.passthroughConn.SetWriteDeadline(...)
	C:/gopath/src/github.com/denisenkom/go-mssqldb/net.go:167
crypto/tls.(*Conn).SetWriteDeadline(...)
	c:/go116/src/crypto/tls/conn.go:150
(snip)

#642 maybe fix it.

[robinknaapen]

I've added Go116 pipeline to #642

[denisenkom]

This is done in a different PR