Update CHANGELOG.md

Author: Bill-Becker

CHANGELOG.md

@@ -25,6 +25,11 @@ Classify the change according to the following categories:
     ### Deprecated
     ### Removed
 
+## test-runners
+### Added
+- Memory-clearing commands after each JuMP model instance in `runtests.jl` to avoid memory buildup which were slowing down Actions test job
+- Added back `ubuntu` OS as an additional runner OS for the tests Action job, now that memory buildup is reduced (removed a year ago due to memory crashing the runner)
+
 ## v0.52.0
 ### Added
 - Add **Financial** inputs `min_initial_capital_costs_before_incentives` and `max_initial_capital_costs_before_incentives` which, when provided, provide upper and lower bounds on initial capital costs for all technologies.