refactor(build): Remove Release Phase and only use Version

`Version` is SemVer so we can add release candidate and other
designations as required.

Continue to use `dev` as default version, overridable by building with
ldflag set.

Dockerfile

@@ -1,15 +1,13 @@
 FROM golang:1.15 as build
-ARG RELEASE_PHASE
 ARG VERSION
 
 WORKDIR /app
 COPY ./ ./
 
-ENV LD_RELEASE_PHASE="-X github.com/spinnaker/spin/version.ReleasePhase=${RELEASE_PHASE}"
 ENV LD_VERSION="-X github.com/spinnaker/spin/version.Version=${VERSION}"
 
 RUN GOARCH=amd64 GOOS=linux CGO_ENABLED=0 go build \
-    -ldflags "${LD_VERSION} ${LD_RELEASE_PHASE}" .
+    -ldflags "${LD_VERSION}" .
 
 FROM alpine
 

version/version.go

@@ -14,25 +14,13 @@
 
 package version
 
-import (
-	"fmt"
-)
-
 // Main version number being run right now.
-var Version = ""
-
-// A pre-release marker for the version. If this is an empty string,
-// then the release is a final release. Otherwise this is a pre-release
-// version e.g. "dev", "alpha", etc.
-var ReleasePhase = "dev"
+var Version = "dev"
 
 // User Agent name set in requests.
 const UserAgent = "Spin-CLI"
 
 // String prints the version of the Spin CLI.
 func String() string {
-	if ReleasePhase != "" {
-		return fmt.Sprintf("%s-%s", Version, ReleasePhase)
-	}
 	return Version
 }