v0.15.0 (#3627)

Also updates the README to:
 - highlight Dendrite is in maintenance mode
- remove references to being "scalable" as we eventually decided to
focus on smaller deployments not huge ones.
- remove the progress section as it was horribly outdated and in general
Dendrite is roughly feature complete with synapse (with the exception of
sliding sync and OIDC, which is also now mentioned explicitly)

internal/version.go

@@ -17,8 +17,8 @@ var build string
 
 const (
 	VersionMajor = 0
-	VersionMinor = 14
-	VersionPatch = 1
+	VersionMinor = 15
+	VersionPatch = 0
 	VersionTag   = "" // example: "rc1"
 
 	gitRevLen = 7 // 7 matches the displayed characters on github.com