[1.0] ci: add Go 1.17, drop Go 1.15

Add Go 1.17, grop 1.15 from CI; since 1.17 release 1.15 is unsupported.

Keep 1.13 in 1.0 branch, since an older version of Docker/Moby might use
it.

Keep 1.16 in Dockerfile, since this is a stable branch and we'd rather
not swap horses in the middle of the stream.

This corresponds to commit a587180 in main branch.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         # Docker/Moby still builds runc with Go 1.13, so we should still support Go 1.13.
-        go-version: [1.13.x, 1.15.x, 1.16.x]
+        go-version: [1.13.x, 1.16.x, 1.17.x]
         rootless: ["rootless", ""]
         race: ["-race", ""]