Update MbedTLS to 2.26.0

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: Idfa92383d03f70bb7af864b7b8d35ca66102cbca
diff --git a/tf-m-nightly-code-coverage.yaml b/tf-m-nightly-code-coverage.yaml
index ccf7897..2e79752 100644
--- a/tf-m-nightly-code-coverage.yaml
+++ b/tf-m-nightly-code-coverage.yaml
@@ -61,7 +61,7 @@
             For use when triggering with unmerged Gerrit changes.
       - string:
           name: 'MBEDTLS_VERSION'
-          default: 'mbedtls-2.25.0'
+          default: 'mbedtls-2.26.0'
       - string:
           name: 'MBEDTLS_URL'
           default: 'https://git.trustedfirmware.org/mirror/mbed-tls.git'