.github/workflows: add Go 1.17 to test matrix
.github/workflows: add Go 1.17 to test matrix

Drop 1.15 and keep 1.12, 1.16.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

go4org/netipx