Gracefully drop MSC2716 related tables (incrementally import history)

[MadLittleMods]

Gracefully drop MSC2716 related tables (incrementally import history) in multiple steps/phases.

Step 1

In one Synapse version, stop writing to any MSC2716 related tables. The normal steps to gracefully drop a table usually include a step to first switch over reads but keep writing but since this is a experimental feature, we don't need to worry about the feature working we can just stop reading and writing in the same step.

This is being tackled in #15748

Step 2

In the Synapse release after the first step ships, we can bump SCHEMA_COMPAT_VERSION and drop the tables altogether.

Relevant tables to drop:

-- Drop tables related to MSC2716 since the implementation is being removed
DROP TABLE insertion_events;
DROP TABLE insertion_event_edges;
DROP TABLE insertion_event_extremities;
DROP TABLE batch_events;

[clokep]

See #16465