release-23.2: ui: keep timewindow with fixed range after Now button click

[blathers-crl]

Backport 1/1 commits from #115044 on behalf of @koorosh.

/cc @cockroachdb/release

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This change fixes behavior of Now button in TimeScale component. Before, when user selected some custom time range then clicked Now button - it would not update charts because there was inconsistency. Custom time range cannot be with "moving" attribute, it cannot be updated every time to reflect most recent data as custom range has fixed end time.

This change checks if current preset is custom and doesn't set "moving" attribute in this case.

Epic: None

Resolves: #113091

Release note (ui change): proper handling of "Now" button on Time range selector when custom time period was selected before.

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Release justification: low risk, high benefit changes to existing functionality

[blathers-crl]

Thanks for opening a backport.

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[cockroach-teamcity]

This change is