modules/understanding-upgrade-channels: Drop nightly discussions

We've had this section since back in 42c1006 (Move KB article
"OpenShift 4.2 Upgrades phased roll out" contents into docs,
2020-01-10, #18944), but there are a few wrinkles around nightlies:

* The canonical release controller URIs are now given on [1], and they
  prefer [2] and similar pretty URIs instead of the l2s4 business in
  the paragraph I'm removing.

* On the release controllers, all single-arch nightlies list
  CI-registry pullspecs, e.g. [3]:

    oc adm release extract --tools registry.ci.openshift.org/ocp/release:4.12.0-0.nightly-2022-09-16-035656

* CI-registry access requires Red Hat single-sign-on [4] and
  membership in the Red Hat Rover group or similar [5].  This is not a
  customer-facing option.

* ART promotes a subset of release-controller-accepted nightlies are
  promoted to the mirrors like [6].  These have quay.io/ pullspecs and
  are customer-accessible.

* But ART stops promoting 4.y nightlies to the mirrors when they
  release the first feature candidate for the 4.y (4.y.0-fc.#).  And
  beginning in 4.12, ART is no longer pushing nightlies to mirrors at
  all, since we started cutting sprintly builds (engineering
  candidates):

    $ w3m -dump -cols 200 https://mirror.openshift.com/pub/openshift-v4/amd64/clients/ocp-dev-preview/ | grep '4[.]12[.]'
     4.12.0-0.nightly-2022-07-02-041854  —    -
     4.12.0-0.nightly-2022-07-05-083442  —    -
     4.12.0-0.nightly-2022-07-05-155211  —    -
     4.12.0-0.nightly-2022-07-05-191634  —    -
     4.12.0-0.nightly-2022-07-05-225149  —    -
     4.12.0-0.nightly-2022-07-06-023534  —    -
     4.12.0-0.nightly-2022-07-06-061520  —    -
     4.12.0-0.nightly-2022-07-06-094950  —    -
     4.12.0-0.nightly-2022-07-06-134516  —    -
     4.12.0-0.nightly-2022-07-06-221008  —    -
     4.12.0-0.nightly-2022-07-07-092951  —    -
     4.12.0-0.nightly-2022-07-07-144231  —    -
     4.12.0-0.nightly-2022-07-11-054352  —    -
     4.12.0-0.nightly-2022-07-25-055755  —    -
     4.12.0-ec.0                         —    -
     4.12.0-ec.1                         —    -
     4.12.0-ec.2                         —    -
     4.12.0-ec.3                         —    -

Removing the nightly reference avoids sending customers down this
not-very-useful-for-them rabbit hole.  A more reliable location for
learning about things outside of candidate channels is [7], but we
don't need to link to that or discuss it from the upgrade-channel
documentation.

[1]: https://docs.ci.openshift.org/docs/getting-started/useful-links/#services
[2]: https://amd64.ocp.releases.ci.openshift.org/
[3]: https://amd64.ocp.releases.ci.openshift.org/releasestream/4.12.0-0.nightly/release/4.12.0-0.nightly-2022-09-16-035656
[4]: https://docs.ci.openshift.org/docs/how-tos/use-registries-in-build-farm/#how-do-i-log-in-to-pull-images-that-require-authentication
[5]: https://docs.ci.openshift.org/docs/how-tos/rbac/#rover-groups
[6]: https://mirror.openshift.com/pub/openshift-v4/amd64/clients/ocp-dev-preview/
[7]: https://console.redhat.com/openshift/install/pre-release

Author: wking

modules/understanding-upgrade-channels.adoc

@@ -18,18 +18,6 @@ The `candidate-{product-version}` channel contains candidate builds for a z-stre
 
 You can use the `candidate-{product-version}` channel to update from a previous minor version of {product-title}.
 
-[NOTE]
-====
-Release candidates differ from the nightly builds. Nightly builds are available for early access to features, but updating to or from nightly builds is neither recommended nor supported. Nightly builds are not available in any upgrade channel. You can reference the {product-title}
-ifdef::openshift-origin[]
-link:https://origin-release.apps.ci.l2s4.p1.openshiftapps.com/[release statuses]
-endif::[]
-ifndef::openshift-origin[]
-link:https://openshift-release.apps.ci.l2s4.p1.openshiftapps.com/[release statuses]
-endif::[]
-for more build information.
-====
-
 [id="fast-version-channel_{context}"]
 == fast-{product-version} channel