Add remove-sqlalchemy-migrate

Delete the legacy sqlalchemy-migrate-based migrations and related
tooling.

Part of blueprint remove-sqlalchemy-migrate

Change-Id: I88e64043f268b857de69f98a78c74c1ac6b02120
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

specs/xena/approved/glance/remove-sqlalchemy-migrate.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+====================================
+Spec Lite: Remove sqlalchemy-migrate
+====================================
+
+..
+  Mandatory sections
+
+:project: glance
+
+:problem: The sqlalchemy-migrate library is unmaintained and has been
+          deprecated in favour of alembic. It is not compatible with sqlalchemy
+          2.0 and is issuing many warnings suggesting Python 3.10 compatibility
+          may be in doubt. Glance switched to alembic for new migrations during
+          the Ocata cycle and no longer needs to carry the old
+          sqlalchemy-migrate-based migrations.
+
+:solution: Remove the sqlalchemy-migrate-based migrations and related tooling.
+
+:impacts: None. Users coming from a pre-Ocata deployment will need to deploy a
+          pre-Xena glance before deploying Xena, but that's standard practice
+          anyway.
+
+..
+  Optional sections -- delete any that don't apply to this spec lite
+
+:timeline: Xena milestone 3
+
+:link: https://review.opendev.org/c/openstack/glance/+/760411
+
+:assignee: stephenfin