Deprecate the release tag

It is optional now, and considered GA when not set.

Semver pre-release tags should be used now to publish non-ga versions.
See elastic#225

Author: jsoriano

versions/1/changelog.yml

@@ -7,6 +7,9 @@
   - description: Prepare for 1.3.1
     type: enhancement
     link: https://github.com/elastic/package-spec/pull/250
+  - description: Deprecate the release attribute, make it optional and set to ga by default
+    type: enhancement
+    link: https://github.com/elastic/package-spec/issues/225
 - version: 1.3.0
   changes:
   - description: Support wait-for-data timeout

versions/1/data_stream/manifest.spec.yml

@@ -115,6 +115,7 @@ spec:
       - AWS billing metrics
     release:
       description: Stability of data stream.
+      deprecated: true # See https://github.com/elastic/package-spec/issues/225
       type: string
       enum:
       - experimental

versions/1/manifest.spec.yml

@@ -145,11 +145,13 @@ spec:
       - integration
     release:
       description: The stability of the package.
+      deprecated: true # See https://github.com/elastic/package-spec/issues/225
       type: string
       enum:
       - experimental
       - beta
       - ga
+      default: ga
       examples:
       - experimental
     categories:
@@ -295,5 +297,4 @@ spec:
   - title
   - description
   - version
-  - release
   - owner