Go unstable=1.19, stable=1.18, oldstable=1.17

[atc0005]

CHANGES

• each image type (stable, unstable, oldstable) has their own
  copy of the golangci-lint config file with the stable variants
  being given a copy of the via the Makefile build recipe
• each image type (stable, unstable, oldstable) is given a copy
  of the markdownlint config file via Makefile build recipe
• Makefile clean recipe updated to remove temporary copies of linter
  config files
• Makefile build recipe updated to limit copying of linter config
  files to just the images which use them
• oldstable image
  • explicitly specifies the Go version limit for golangci-lint as Go
    1.17
  • updated Go from 1.16.15 to 1.17.11
• unstable image
  • explicitly specifies the Go version limit for golangci-lint as Go
    1.18 (for now)
  • updated Go from 1.18.3 to 1.19beta1
• stable image
  • variants providing the golangci-lint linter explicitly specify the
    Go version limit for golangci-lint as Go 1.18
  • updated Go from 1.17.11 to 1.18.3
• update README to drop compatibility notes regarding the oldstable
  image and older staticcheck version used for Go 1.16 compatibility
• update go.mod file to reflect current stable version of Go (1.18)
  with a note to update the version going forward

refs GH-557
refs GH-656