docs: Update changelog for v1.6.0 release (#1920)

Preparing for the v1.6.0 release. Removed changelog for Docker-based
builder since we haven't officially released it yet.

Signed-off-by: Ian Lewis <[email protected]>
Co-authored-by: asraa <[email protected]>

Author: ianlewis

CHANGELOG.md

@@ -11,18 +11,15 @@
     - [Generic generator](#generic-generator)
       - [New Features](#new-features-1)
     - [Container generator](#container-generator)
-      - [New Features](#new-features-2)
-    - [Docker-based generator](#docker-based-generator)
-      - [New Features](#new-features-3)
   - [Changelog since v1.5.0](#changelog-since-v150)
 - [v1.5.0](#v150)
   - [Summary of changes](#summary-of-changes-1)
     - [Go builder](#go-builder-1)
-      - [New Features](#new-features-4)
+      - [New Features](#new-features-2)
     - [Generic generator](#generic-generator-1)
-      - [New Features](#new-features-5)
+      - [New Features](#new-features-3)
     - [Container generator](#container-generator-1)
-      - [New Features](#new-features-6)
+      - [New Features](#new-features-4)
   - [Changelog since v1.4.0](#changelog-since-v140)
 - [v1.4.0](#v140)
   - [What's Changed](#whats-changed)
@@ -88,10 +85,10 @@
   [`prerelease`](https://github.com/slsa-framework/slsa-github-generator/blob/v1.6.0/internal/builders/generic/README.md#workflow-inputs)
   input was added to allow users to create releases marked as prerelease when
   `upload-assets` is set to `true`.
-- A new output `go-provenance-name` added which can be used to retrieve the name
-  of the provenance file generated by the builder.
 - A new input `draft-release` was added to allow users to create releases marked
   as draft when `upload-assets` is set to `true`.
+- A new output `go-provenance-name` added which can be used to retrieve the name
+  of the provenance file generated by the builder.
 
 #### Generic generator
 
@@ -102,14 +99,8 @@
 
 #### Container generator
 
-##### New Features
-
-#### Docker-based generator
-
-##### New Features
-
-- A new input `draft-release` was added to allow users to create releases marked
-  as draft when `upload-assets` is set to `true`.
+The Container Generator was updated to use `cosign` v2.0.0. No changes to the
+workflow's inputs or outputs were made.
 
 ### Changelog since v1.5.0