version: Update version following 5.0 release.

docs/overview/changelog.md

@@ -1,9 +1,19 @@
 # Version history
 
-This page contains the release history for the Zulip 5.x stable
-release series. See the [current Zulip changelog][latest-changelog]
-for newer release series, or the [commit log][commit-log] for an
-up-to-date list of raw changes.
+This page the release history for the Zulip server.  See also the
+[Zulip release lifecycle](../overview/release-lifecycle.md).
+
+## Zulip 6.x series
+
+### 6.0 -- unreleased
+
+This section is an incomplete draft of the release notes for the next
+major release, and is only updated occasionally.  See the [commit
+log][commit-log] for an up-to-date list of raw changes.
+
+#### Upgrade notes for 6.0
+
+- None yet.
 
 ## Zulip 5.x series
 
@@ -2580,6 +2590,7 @@ running a version from before 1.7 should upgrade directly to 1.7.1.
 This section links to the upgrade notes from past releases, so you can
 easily read them all when upgrading across multiple releases.
 
+* [Draft upgrade notes for 6.0](#upgrade-notes-for-6-0)
 - [Upgrade notes for 5.0](#upgrade-notes-for-50)
 - [Upgrade notes for 4.0](#upgrade-notes-for-40)
 - [Upgrade notes for 3.0](#upgrade-notes-for-30)

templates/zerver/api/changelog.md

@@ -18,6 +18,11 @@ clients should check the `zulip_feature_level` field, present in the
 /register`](/api/register-queue) responses, to determine the API
 format used by the Zulip server that they are interacting with.
 
+## Changes in Zulip 6.0
+
+Feature levels 123-124 are reserved for future use in 5.x maintenance
+releases.
+
 ## Changes in Zulip 5.0
 
 **Feature level 122**

version.py

@@ -1,6 +1,6 @@
 import os
 
-ZULIP_VERSION = "5.0"
+ZULIP_VERSION = "6.0-dev+git"
 
 # Add information on number of commits and commit hash to version, if available
 zulip_git_version_file = os.path.join(