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

.ci/scripts/calculate_jobs.py

@@ -99,24 +99,24 @@ set_output("trial_test_matrix", test_matrix)
 
 # First calculate the various sytest jobs.
 #
-# For each type of test we only run on bullseye on PRs
+# For each type of test we only run on bookworm on PRs
 
 
 sytest_tests = [
     {
-        "sytest-tag": "bullseye",
+        "sytest-tag": "bookworm",
     },
     {
-        "sytest-tag": "bullseye",
+        "sytest-tag": "bookworm",
         "postgres": "postgres",
     },
     {
-        "sytest-tag": "bullseye",
+        "sytest-tag": "bookworm",
         "postgres": "multi-postgres",
         "workers": "workers",
     },
     {
-        "sytest-tag": "bullseye",
+        "sytest-tag": "bookworm",
         "postgres": "multi-postgres",
         "workers": "workers",
         "reactor": "asyncio",
@@ -127,11 +127,11 @@ if not IS_PR:
     sytest_tests.extend(
         [
             {
-                "sytest-tag": "bullseye",
+                "sytest-tag": "bookworm",
                 "reactor": "asyncio",
             },
             {
-                "sytest-tag": "bullseye",
+                "sytest-tag": "bookworm",
                 "postgres": "postgres",
                 "reactor": "asyncio",
             },