ci: review linters

[gurza]

• replace deprecated varcheck, deadcode, and structcheck linters by unused
• remove duplicated entity (megacheck linter) from linters list

[gurza]

Also, I noticed that there are linters in the linters-settings configuration that are not in linters configuration.

• maligned
• goconst
• lll

They must either be removed from the linters-settings configuration or added to the linters configuration.

[paskal]

For that PR, please also bump the golangci-lint version in https://github.com/go-pkgz/email/blob/master/.github/workflows/ci.yml#L35