chore(sdk): release KFP SDK 2.8.0

[chensun]

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[google-oss-prow]

@chensun: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kfp-kubernetes-execution-tests	f86ef32	link	false	/test kfp-kubernetes-execution-tests

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[gregsheremeta]

#10913 is sad that it can't join the party

[chensun]

/retest-required

[chensun]

#10913 is sad that it can't join the party

I can do another release once the PR is merged. Given there's backend change involved, SDK release should come after backend release.

[google-oss-prow]

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