channels/stable-4.3: Add 4.2.16 through 4.2.19 (amd64)

It's been at least 24 days since the fast promotions landed, so finish
the "phased rollout" and put them in stable.

* 4.2.16+amd64 was promoted to the feeder fast-4.3 by 7660e1d
  (Merge pull request openshift#73 from wking/4.2.16-to-fast-4.3, 2020-02-21).
* There was no 4.2.17 (no releases in the week after 4.2.16).
* 4.2.18+amd64 was promoted to the feeder fast-4.3 by fa66fd4
  (Merge pull request openshift#60 from wking/fast-4.2.18, 2020-02-20).
* 4.2.19+amd64 was promoted to the feeder fast-4.3 by 04c5b05
  (Merge pull request openshift#64 from thiagoalessio/fast-4.2, 2020-02-24).

The previous comments about "only valid into" were from 505f7a6
(Add 4.2 edges into stable-4.3, 2020-03-12, openshift#109), and I'm not clear
on the motivation.

channels/stable-4.3.yaml

@@ -1,9 +1,9 @@
 name: stable-4.3
 versions:
-# 4.2.16+amd64 - withheld as it's only valid into 4.3.0,4.3.1 people should go direct to 4.3.5
+- 4.2.16+amd64
 # not 4.2.17 because we had a long quiet time after 4.2.16 with no releases
-# 4.2.18+amd64 - withheld as it's only valid into 4.3.1, people should go direct to 4.3.5
-# 4.2.19+amd64 - withheld as it's only valid into 4.3.1, people should go direct to 4.3.5
+- 4.2.18+amd64
+- 4.2.19+amd64
 - 4.2.20+amd64
 - 4.2.21+amd64
 - 4.2.22+amd64