Add exclude for tryBase64Decode to backward compat test (follow-up to #56913)

[rschu1ze]

Unflakes test_backward_compatibility/test_functions.py::test_string_functions

Fixes #56969

Changelog category (leave one):

• Not for changelog (changelog entry is not required)

[robot-ch-test-poll]

This is an automated comment for commit a3c9f13 with description of existing statuses. It's updated for the latest CI running

⏳ Click here to open a full report in a separate page

Successful checks

Check name	Description	Status
Mergeable Check	Checks if all other necessary checks are successful	✅ success
Push to Dockerhub	The check for building and pushing the CI related docker images to docker hub	✅ success

Check name	Description	Status
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	⏳ pending