Explicitly mark earliest-version for release notes

This uses reno's `earliest-version` directive to explicitly point out
where the release notes for a given branch should begin. There is a bug
in reno when dealing with stable branches, where it does not always find
the correct version to begin with.

It also updates our release docs to indicate we should continue marking
this. This step can be removed when the reno bug is fixed, though it
doesn't hurt to continue marking this explicitly.

Change-Id: I6502ff95a52c2c855356e9875291f27ec44e7ffa
Related-Bug: #1746076

doc/source/contributor/releasing.rst

@@ -85,7 +85,10 @@ This includes:
   * updating the release notes RST to include the new branch.
 
     The generated RST does not include the version range in the title, so we
-    typically submit a follow-up patch to do that.
+    typically submit a follow-up patch to do that. We also manually mark
+    the ``earliest-version`` directive on the new page, due to a `reno bug
+    <https://bugs.launchpad.net/reno/+bug/1746076>` that may cause this to
+    be incorrect for stable branches.
 
 We need to submit patches for changes in the stable branch to:
 

releasenotes/source/liberty.rst

@@ -4,6 +4,7 @@ Liberty Series (4.0.0 - 4.2.5) Release Notes
 
 .. release-notes::
    :branch: origin/stable/liberty
+   :earliest-version: 4.2.2
 
 .. _V4-2-1:
 

releasenotes/source/mitaka.rst

@@ -4,3 +4,4 @@ Mitaka Series (4.3.0 - 5.1.x) Release Notes
 
 .. release-notes::
    :branch: origin/stable/mitaka
+   :earliest-version: 4.3.0

releasenotes/source/newton.rst

@@ -4,3 +4,4 @@ Newton Series (6.0.0 - 6.2.x) Release Notes
 
 .. release-notes::
    :branch: origin/stable/newton
+   :earliest-version: 6.0.0

releasenotes/source/ocata.rst

@@ -4,3 +4,4 @@ Ocata Series (7.0.0 - 7.0.x) Release Notes
 
 .. release-notes::
    :branch: origin/stable/ocata
+   :earliest-version: 7.0.0

releasenotes/source/pike.rst

@@ -4,3 +4,4 @@
 
 .. release-notes::
    :branch: stable/pike
+   :earliest-version: 8.0.0