Update changelog for PR thephpleague#1236

Updated the changelog to add details about PR thephpleague#1236. This PR allows us
to support version 4.1.x or greater of lcobucci/jwt by conditionally
supporting the StrictValidAt() validator.

Author: Sephster

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Changed
+- Conditionally support the `StrictValidAt()` method in lcobucci/jwt so we can use version 4.1x of the library (PR #1236)
 
 ## [8.3.1] - released 2021-06-04
 ### Fixed