According to ISTQB (v2.3) "Standard glossary of terms used in Software Testing":
Test approach: The implementation of the test strategy for a specific project. It typically includes the decisions made that follow
based on the (test) project’s goal and the risk ATM assessment carried
out, starting points regarding the test process, the test design
techniques to be applied, exit criteria and test types to be
Test technique (Test design technique): Procedure used to derive and/or select test cases.
So the difference: approach is more project based and technique is more fundamental term, we define test techniques to be used in our test approach.
E.g.: Test approach is about project's scope, objectives, test types and tools, also about applied test techniques: structure-based, specification-based, people-based, coverage-based, problems-based, activity-based, evaluation-based whatever.
Also Exploratory testing is a test design technique as well as Boundary value analysis (BVA), Structure-based testing (context-based, less used term).