Cleanup istio-testing GCS buckets

[howardjohn]

Currently using a LOT of data

artifacts.istio-testing.appspot.com: 39.933TiB - this is gcr.io/istio-testing, contains our per-commit builds among others

Old (1.3) daily/monthly releases
istio-presubmit-release-pipeline-data: 33.151TiB - artifacts
artifacts.istio-prerelease-testing.appspot.com: 4.947TiB - docker images

Prow logs
istio-prow: 17.794TiB

Proxy builds
istio-build: 15.457TiB
istio-artifacts: 11.007TiB

Old 2017 logs
e2e-testing-log: 887.949GiB
istio-logs: 200gb

The most likely candidates here are the old monthly builds and the old prow logs. Both of these may not be worth the cost of removal though.

If we really wanted to optimize space, which I don't think we need to, the istio-build/istio-artifacts probably has some junk, but previously we accidentally deleted some important files there so probably best not to touch it

[chizhg]

If we are certain the old monthly builds and old prow logs are not needed, shall we just configure retention policy?

[howardjohn]

This has still not been done, probably useful to do