Always treat RETURNING as supported by SQL engines (#19047)

Can do this now that SQLite 3.35.0 added support for `RETURNING`.

> The RETURNING syntax has been supported by SQLite since version 3.35.0
(2021-03-12).
>
> *-- https://sqlite.org/lang_returning.html*

This also bumps the minimum supported SQLite version according to
Synapse's [deprecation
policy](https://element-hq.github.io/synapse/latest/deprecation_policy.html#platform-dependencies).

Fix https://github.com/element-hq/synapse/issues/17577

.github/workflows/latest_deps.yml

@@ -139,9 +139,9 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - sytest-tag: bullseye
+          - sytest-tag: bookworm
 
-          - sytest-tag: bullseye
+          - sytest-tag: bookworm
             postgres: postgres
             workers: workers
             redis: redis

.github/workflows/twisted_trunk.yml

@@ -108,11 +108,11 @@ jobs:
     if: needs.check_repo.outputs.should_run_workflow == 'true'
     runs-on: ubuntu-latest
     container:
-      # We're using debian:bullseye because it uses Python 3.9 which is our minimum supported Python version.
+      # We're using bookworm because that's what Debian oldstable is at the time of writing.
       # This job is a canary to warn us about unreleased twisted changes that would cause problems for us if
       # they were to be released immediately. For simplicity's sake (and to save CI runners) we use the oldest
       # version, assuming that any incompatibilities on newer versions would also be present on the oldest.
-      image: matrixdotorg/sytest-synapse:bullseye
+      image: matrixdotorg/sytest-synapse:bookworm
       volumes:
         - ${{ github.workspace }}:/src