Target v2.10-head for e2e runs

[rak-phillip]

Summary

head recently introduced a breaking that causes several e2e test failures. This is a fix for release-2.10 to target 2.10-head, which should be the targeted version for this release moving forward. We originally didn't target 2.10-head at the time of branching for Dashboard because Rancher core did not yet have 2.10-head image available for use.

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[richard-cox]

There's some major flakyness going on with the e2e tests. Looks mostly due to rancher not coming up generally, or fleet not coming up in time. Both of those were improved by recent flaky fixes in head, so created #12575 which contains the backport and this PR. Will see if that improves the tests 🤞

[rak-phillip]

@richard-cox thanks for picking up the fix for the flaky tests. I'm closing this PR in favor of getting #12575 merged.