Add nightly job for the mbedtls-3.6 branch

Change-Id: I093892ed02b60748e0c27aaf637e4185c6ea472f
diff --git a/mbed-tls-nightly-tests.yaml b/mbed-tls-nightly-tests.yaml
index abf1863..a86becf 100644
--- a/mbed-tls-nightly-tests.yaml
+++ b/mbed-tls-nightly-tests.yaml
@@ -138,4 +138,7 @@
         properties-content: GIT_CREDENTIALS_ID=mbedtls-github-token
     triggers:
     - parameterized-timer:
-        cron: "H(0-29) 0 * * * %MBED_TLS_BRANCH=development \nH(0-29) 2 * * * %MBED_TLS_BRANCH=mbedtls-2.28"
+        cron: |
+          H(0-29) 0 * * * %MBED_TLS_BRANCH=development
+          H(0-29) 2 * * * %MBED_TLS_BRANCH=mbedtls-3.6
+          H(0-29) 4 * * * %MBED_TLS_BRANCH=mbedtls-2.28