[www] Set Yoga series state as Unmaintained

With the new TC resolution the community replaces Extended Maintenance
with Unmaintained status [1].

According to the resolution the
- tip of the stable/yoga branches were tagged with yoga-eom tag
- unmaintained/yoga branches were cut from yoga-eom tags
- stable/yoga branches were deleted

Some further notes:
- there will no longer be official releases off of yoga branch
- backports can be pushed against unmaintained/yoga branch
- openstack-unmaintained-core have the rights to merge patches
  to unmaintained/* branches

For further details please read the resolution [1].

[1] https://governance.openstack.org/tc/resolutions/20230724-unmaintained-branches.html

Change-Id: I789225f402454e4cdd64118e5adaeb35368b7269

tools/www-generator.py

@@ -72,7 +72,7 @@ SERIES_INFO = collections.OrderedDict([
         SeriesInfo(date='October 2020', status='extended-maintenance')),
     ('wallaby', SeriesInfo(date='April 2021', status='extended-maintenance')),
     ('xena', SeriesInfo(date='October 2021', status='extended-maintenance')),
-    ('yoga', SeriesInfo(date='April 2022', status='maintained')),
+    ('yoga', SeriesInfo(date='April 2022', status='unmaintained')),
     ('zed', SeriesInfo(date='October 2022', status='maintained')),
     ('2023.1', SeriesInfo(date='March 2023', status='maintained')),
     ('2023.2', SeriesInfo(date='October 2023', status='maintained')),