Updated verbiage for backfill runs being deprioritized

[g-dailey]

Adds UI messaging to inform users that backfill DAG runs are
deprioritized behind scheduled and manual runs in the scheduler loop.

Backfill runs have always been lower priority than non-backfill runs due
to how the scheduler orders DAG runs each cycle, but this was not
communicated anywhere in the UI. Users were left confused when backfill
runs remained in a queued/scheduled state longer than expected.

This PR surfaces that context in two places:

• An info alert on the Run Backfill form, shown before a user triggers a backfill
• The backfill in progress banner, so users are reminded while a backfill is running

This is what the new banner looks like:

Closes: #66729

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[bbovenzi]

I think it would be helpful to put this in a tooltip. It takes up a lot of space in the banner.

[potiuk]

@g-dailey — There is 1 unresolved review thread on this PR from bbovenzi. Could you either push a fix or reply in the thread explaining why the feedback doesn't apply? When you believe the feedback is addressed, please mark the thread as resolved and ping the reviewer (bbovenzi) for a final look. Thanks!

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[pierrejeambrun]

Good idea.

Just one suggestion from Brent to try to make save some real estate on the page but other than this that's a nice improvement :)

[g-dailey]

Thank you all for the feedback, just updated the longer description to go in a tooltip.

Tooltip is used on this page:

[pierrejeambrun]

LGTM, thanks

[pierrejeambrun]

Static check are failing and need fixing. (imports orders)