Add deprecation info for Time To Interactive

[malcomio]

Follows on from #5213

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[chrome-devrel-infra[bot]]

:white_check_mark: Preview (static) for commit d1443a3a487882d3b9d37a6c9790b29f45bc9243 available at https://pr-6409-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-6409-static-dot-dcc-staging.uc.r.appspot.com/docs/lighthouse/performance/interactive/