x/pkgsite: migrate CI from Kokoro to LUCI

[h9jiang]

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As discussed with findleyr@, this could be a good candidate for the next FIX IT week so label FRICTION is added.

In golang/vscode-go#3533, we migrated the CI for vscode-go from Kokoro to LUCI. We want to migrate x/pkgsite as well.

From high level, here is what we did to migrate vscode-go from Kokoro to LUCI, there is also an internal document explaining this. But most CLs are being tracked in golang/vscode-go#3533:

• Create builder for x/pkgsite in x/build luci-config branch.
• Enable only the postsubmit in x/pkgsite. (Keep in mind, we do not want any CQ CL 642558, enable CQ means LUCI will be watching presubmit and it will trigger a test and the test will pass immediately)
• Make sure the postsubmit are consistently passing. The development flow will be explained below.
• Enable the presubmit in x/pkgsite.
• Make sure LUCI-TryBot-Result=+1 is a submission requirement. (If its inherit from All-Project, we just need to remove overwriting.) So both LUCI and Kokoro +1 are required for submission.
• Soak the env for a few weeks, remove the Kokoro +1 from submission requirement and remove Kokoro config.

When post submit are enabled, we might see test failing. So we need to submit CLs to fix the post submit test. However, we can only know if a CL is fixing the test after submission (because it's triggered as post submit). There is a way to trigger the post submit against an open CL

led get-builder golang/ci/x_vscode-go-go1.23-linux-amd64_docker | led edit-gerrit-cl https://go-review.git.corp.google.com/c/vscode-go/+/642917 | led launch

where golang/ci/x_vscode-go-go1.23-linux-amd64_docker is the builder name, https://go-review.git.corp.google.com/c/vscode-go/+/642917 is the link to the CL. after executing this command, a LUCI test will be triggered against the code in the CL. So we will know the result before we merge the code.