build: bump the version to 2021.2.0-stable

Author: PenghaiZhang

build.sbt

@@ -116,7 +116,7 @@ name := "Equella"
 (ThisBuild / equellaMajor) := 2021
 (ThisBuild / equellaMinor) := 2
 (ThisBuild / equellaPatch) := 0
-(ThisBuild / equellaStream) := "RC1"
+(ThisBuild / equellaStream) := "Stable"
 (ThisBuild / equellaBuild) := buildConfig.value.getString("build.buildname")
 (ThisBuild / buildTimestamp) := Instant.now().getEpochSecond