Add changelog

Author: lahsivjar

CHANGELOG.asciidoc

@@ -25,6 +25,7 @@ https://github.com/elastic/apm-aws-lambda/compare/v1.4.0...main[View commits]
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 ===== Bug fixes
 - Log a warning, instead of failing a Lambda function, if auth retrieval from AWS Secrets Manager fails. Reporting APM data will not work, but the Lambda function invocations will proceed. {lambda-pull}401[401]
+- Fix incorrect proxy transaction handling at shutdown due to not flushing the data before processing shutdown event. {lambda-pull}412[412].
 
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 ===== Features