Upgrade tfx version to 0.26.0 in backend

[zijianjoy]

Description of your changes:
As the recent update of python 3.7 to fix presubmit tests #5033, upgrade the backend dependencies to be compatible with new python version. The workable solution is to use tfx 0.26.0.

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[chensun]

/approve

Wait for @numerology to take a look and LGTM.

@numerology, a question for you, Per this README, it looks like this dependency is only for visualization server? If that's the case, should we merge this requirements.* with the ones under https://github.com/kubeflow/pipelines/tree/master/backend/src/apiserver/visualization?

[k8s-ci-robot]

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[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun, zijianjoy

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• backend/OWNERS [chensun]

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[numerology]

/lgtm

Thanks!
@chensun this requirements file is actually for the pre-baked sample, which is built into our backend docker image. See

pipelines/backend/Dockerfile

Line 31 in d14f64e

# Compiling the preloaded samples.