add 1.26.1 release to changelog

The 1.26.1 release is a branched release from the 1.26.0 version, with log4j upgraded to eliminate the shell vulnerability - for customers who are finding issues with more recent versions

Author: jackshirazi

CHANGELOG.asciidoc

@@ -172,6 +172,14 @@ discovery - {pull}2205[#2205]
 * Replace concurrency plugin maps to `SpanConcurrentHashMap` ones - {pull}2173[#2173]
 * Align User-Agent HTTP header with other APM agents - {pull}2177[#2177]
 
+[[release-notes-1.26.1]]
+==== 1.26.1 - 2021/12/22
+
+[float]
+===== Dependency updates
+* Update Log4j to 2.12.3
+* Update ecs-logging-java to 1.3.0
+
 [[release-notes-1.26.0]]
 ==== 1.26.0 - 2021/09/14