Mark Solum project inactive

This project is not healthy as their gate is broken
and release patches are not merged, see in particular:

https://review.opendev.org/c/openstack/solum/+/894993
https://review.opendev.org/c/openstack/solum/+/895293
https://review.opendev.org/c/openstack/solum/+/895291
https://review.opendev.org/c/openstack/solum/+/895292

There is time till milestone-2 of this release cycle to recover
the situstion and make project healthy and we can remove it from
inactive list. Refer to the below doc to know more about Inactive
state

- https://governance.openstack.org/tc/reference/emerging-technology-and-inactive-projects.html

Project gerrit and zuul stats:

 Validating Gerrit...
 * There are 4 ready for review patches generated within 180 days
 * There are 1 not reviewed patches generated within 180 days
 * There are 2 merged patches generated within 180 days
 * Unreviewed patch rate for patches generated within 180 days is 25.0 %
 * Merged patch rate for patches generated within 180 days is 50.0 %
 *  Here's top 10 owner for patches generated within 180 days
    (Name/Account_ID: Percentage):
    -  OpenStack Release Bot :  25.00%
    -  likui :  25.00%
    -  OpenStack Proposal Bot :  25.00%
    -  Lukas Piwowarski :  25.00%
 Validate Zuul...
 Set buildsets fetch size to 500
 * Repo: openstack/solum-dashboard gate job builds success rate: 100%
 * Repo: openstack/solum-tempest-plugin gate job builds success rate:
   100%
 * Repo: openstack/solum-specs gate job builds success rate: 100%
 * Repo: openstack/solum gate job builds success rate: 90%
 * Repo: openstack/python-solumclient gate job builds success rate: 90%

Change-Id: I12a9a71d2209871bf1be5ab718fcda5c8b2fef42

reference/emerging-technology-and-inactive-projects.rst

@@ -105,3 +105,4 @@ Current Inactive Projects
 * Monasca (inactive since 2024.1 cycle)
 * Sahara (inactive since 2024.1 cycle)
 * Freezer (inactive since 2024.1 cycle)
+* Solum (inactive since 2024.1 cycle)