Update build

[SuperQ]

• Update Go to 1.18
• Update circleci machine image.
• Switch maildev to new upstream image location.
• Update Go modules to 1.17 format.
• Make dependabot monthly to match prometheus/prometheus.

Signed-off-by: SuperQ superq@gmail.com

[SuperQ]

It looks like MailDev completely dropped STARTLS. (maildev/maildev#274)

[gotjosh]

LGTM, but please see my comment re the go version.

[joeblubaugh]

This PR also included a functional change to the email notifier, using x/text to do casing, and x/text panics on some inputs. Here's a stack trace we've seen while running the mimir alertmanager:

/vendor/gopkg.in/yaml.v2/decode.go:270 +0xa7
gopkg.in/yaml%2ev2.(*decoder).callUnmarshaler(0xc20be3f980, 0xc20c116e70, {0x7f170dda42b8, 0xc0430126c0})
/vendor/github.com/prometheus/alertmanager/config/notifiers.go:196 +0x1b9
github.com/prometheus/alertmanager/config.(*EmailConfig).UnmarshalYAML(0xc0430126c0, 0x2340b20?)
	/vendor/golang.org/x/text/cases/cases.go:51
golang.org/x/text/cases.Caser.String(...)
	/vendor/golang.org/x/text/transform/transform.go:650 +0xbe5
golang.org/x/text/transform.String({0x7f170e30ea18, 0xc0002d2a00}, {0xc20fd3aeb8, 0x2})
...
runtime error: slice bounds out of range [11:7]

That wasn't mentioned in the commit, making this a very difficult to track regression.

I'm going to file an issue asking to revert the email notifier changes. I think they should be done in a separate PR, and I don't understand the problem that the casing call is solving.

I also think we can get away without doing Title casing - Email Header names are case insensitive: https://www.rfc-editor.org/rfc/rfc5322#section-1.2.2 (see also: https://stackoverflow.com/questions/6143549/are-email-headers-case-sensitive), so lower-casing every header with strings.ToLower() should be sufficient to check for collisions. so the code could strings.ToLower() everything, which is a simpler transformation.