Define SQLite compat policy (#13728)

changelog.d/13728.doc

+Define Synapse's compatability policy for SQLite versions.

docs/deprecation_policy.md

 Deprecation Policy for Platform Dependencies
 ============================================
 
-Synapse has a number of platform dependencies, including Python and PostgreSQL.
-This document outlines the policy towards which versions we support, and when we
-drop support for versions in the future.
+Synapse has a number of platform dependencies, including Python, Rust, 
+PostgreSQL and SQLite. This document outlines the policy towards which versions 
+we support, and when we drop support for versions in the future.
 
 
 Policy
@@ -17,12 +17,14 @@ Details on the upstream support life cycles for Python and PostgreSQL are
 documented at [https://endoflife.date/python](https://endoflife.date/python) and
 [https://endoflife.date/postgresql](https://endoflife.date/postgresql).
 
-
 A Rust compiler is required to build Synapse from source. For any given release
 the minimum required version may be bumped up to a recent Rust version, and so
 people building from source should ensure they can fetch recent versions of Rust
 (e.g. by using [rustup](https://rustup.rs/)).
 
+The oldest supported version of SQLite is the version
+[provided](https://packages.debian.org/buster/libsqlite3-0) by
+[Debian oldstable](https://wiki.debian.org/DebianOldStable).
 
 Context
 -------
@@ -44,3 +46,8 @@ generally bump their minimum support Rust versions frequently. In general, the
 Synapse team will try to avoid updating the dependency on Rust to the absolute
 latest version, but introducing a formal policy is hard given the constraints of
 the ecosystem.
+
+On a similar note, SQLite does not generally have a concept of "supported 
+release"; bugfixes are published for the latest minor release only. We chose to
+track Debian's oldstable as this is relatively conservative, predictably updated
+and is consistent with the `.deb` packages released by Matrix.org.
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