Chore: tidy for go 1.20

[jamesez]

Go 1.20's test will fail mdmctl because of strings with trailing newlines. This PR removes the newlines, and adds 1.20 to the build matrix.

I also tidied three minor complaints from staticcheck.

[jessepeterson]

i notice a few (most?) of the newline changes are removing newlines from text that is directly output. i.e. it’s for formatting/output style on a console. does your PR maintain that same formatting?

[jamesez]

i notice a few (most?) of the newline changes are removing newlines from text that is directly output. i.e. it’s for formatting/output style on a console. does your PR maintain that same formatting?

I've made all but one out be md5 identical to previous - the previous output had a trailing whitespace on one line.

[korylprince]

LGTM. I also updated the Go versions for CI and the Dockerfile, and updated the CHANGELOG.