Releasing 2.0.0-alpha.4 (#5279)

* Version bump

* Split generate_changelog.sh into separate script

Can be run in the format `generate_changelog.sh $previous_version $version`.

* remove early exit from publish_draft_release.sh

* adding changelog

* ci: change last_github_release to also find pre-releases

Co-authored-by: Martin Pugh <pugh@s3kr.it>

substrate/docs/CHANGELOG.md

@@ -0,0 +1,46 @@
+# Changelog
+
+The format is based on [Keep a Changelog].
+
+[Keep a Changelog]: http://keepachangelog.com/en/1.0.0/
+
+## Unreleased
+
+## 2.0.0-alpha.3 -> 2.0.0-alpha.4
+
+Runtime
+-------
+
+* Move runtime upgrade to `frame-executive` (#5197)
+* Split fees and tips between author and treasury independently (#5207)
+* Refactor session away from needless double_maps (#5202)
+* Remove `secp256k1` from WASM build (#5187)
+* Introduce default-setting prime for collective (#5137)
+* Adds `vested_transfer` to Vesting pallet (#5029)
+* Change extrinsic_count to extrinsic_index in pallet-utility (#5044)
+
+Client
+------
+
+* client/finality-grandpa: Add Prometheus metrics to GossipValidator (#5237)
+* removes use of sc_client::Client from node-transaction-factory (#5158)
+* removes use of sc_client::Client from sc_network (#5147)
+* Use CLI to configure max instances cache (#5177)
+* client/service/src/builder.rs: Add build_info metric (#5192)
+* Remove substrate-ui.parity.io from CORS whitelist (#5142)
+* removes use of sc_client::Client from sc-rpc (#5063)
+* Use 128mb for db cache default (#5134)
+* Drop db-cache default from 1gig to 32mb (#5128)
+* Add more metrics to prometheus (#5034)
+
+API
+---
+
+* Produce block always on updated transaction pool state (#5227)
+* Add `ext_terminate` (#5234)
+* Add ext_transfer call (#5169)
+* ChainSpec trait (#5185)
+* client/authority-discovery: Instrument code with Prometheus (#5195)
+* Don't include `:code` by default in storage proofs (#5179)
+* client/network-gossip: Merge GossipEngine and GossipEngineInner (#5042)
+* Introduce `on_runtime_upgrade` (#5058)