golang: Drop support for Go1.14

This Go release is EOL now.

Don't drop support for Go1.12 and Go1.13 yet. Even though those Go releases are
also EOL since long ago, they are still being used inside SlapOS:

Go1.12 - by gitlab  (/cc @jerome, @alain.takoudjou)
Go1.13 - by galène  (/cc @tomo, @alain.takoudjou)