Upgrade API server to use Tekton version >= 0.19

[fenglixa]

/kind feature

Description:
[A clear and concise description of what your proposal. What problem does it solve?]
Since Tekton version v0.19.0 released, it contains the features (e.g custom task) we may need to use. But seems it was blocked by the issue of:
argoproj/argo-workflows#4425

And the PR provided:
argoproj/argo-workflows#4810

After that, we can upgrade kfp-tekton API Server to Tekton version 0.19.0.

Additional information:
[Miscellaneous information that will assist in solving the issue.]

[Tomcli]

We are trying to remove all the Argo mock api/tests so it doesn't have to depend on the Argo version. Another task for Tekton 0.19 is to update all the k8s function calls to go-client 0.18.x. For example,
https://github.com/tektoncd/pipeline/pull/3319/files#diff-3b3ca39720cb7f55cf1c1529cee802b2f712fcbec78de47484eb3fc5f1d0eed5R34

[fenglixa]

@Tomcli Do you know when we could upgrade tekton to latest version?