fix: [M3-7138] - Fix Create Volume Drawer static pricing copy and misplaced helper text

[mjac0bs]

Description 📝

This PR cleans up some copy for static Volume pricing ($0.10/GB per month) that was previously missed when creating a volume from a Linode's Storage tab. It also corrects a bug with the size field helper text related to when it should display.

These issues were found/fixed in #9601, but due to that PR's size and amount of refactoring, should be fixed in their own PR.

Preview 📷

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How to test 🧪

1. How to setup test environment?
   Check out this PR.
2. How to reproduce the issue (if applicable)?

• Go to http://localhost:3000/linodes/{id}/storage and click the Create Volume button.
• Observe the hardcoded pricing ($0.10/GB) in the helper text.
• Toggle the DC pricing feature flag on.
• Observe the hardcoded pricing ($0.10/GB) in the helper text.
• Observe the floating size field helper text "The size of the new volume in GB".
• Observe the floating size field helper text prompting region selection, which should not display at all in this drawer (we already have the linode's region).
• Create a volume and observe this floating helper text is also present when resizing a volume.

4. How to verify changes?

• Go to http://localhost:3000/linodes/{id}/storage.
• Click the Create Volume button.
• Notice that the floating helper text is gone.
• Observe the hardcoded pricing ($0.10/GB) in the helper text.
• Toggle the DC pricing feature flag on.
• Observe there is not hardcoded pricing ($0.10/GB) in the helper text.
• Create a volume and observe this floating helper text is not present when resizing a volume.
• Check http://localhost:3000/volumes/create and confirm that the size field helper text displays a price when the feature flag is off or once a region has been selected. When the DC pricing feature flag is on and a region is not selected, confirm text is "Select a region to see cost per month.".

6. How to run Unit or E2E tests?
   The Create Volume Drawer relies on getDynamicVolumePrice, which already has a unit test. (yarn test getDynamicVolumePrice)

[mjac0bs]

This change was also made in the big Volumes refactor PR.

I don't see reason why we'd want to keep this text; it was not displaying in the correct place in the drawers.

I also updated the name to priceDisplayText because legacyHelperText felt misleading. We still want the price to display, even after DC pricing changes, as long as a region is selected.

[abailly-akamai]

Agreed. I must have missed this one when working on volumes.

[abailly-akamai]

Great catch! Looking good

Tested copy and display on the linode storage tab and volume creation with and without the flag.

[abailly-akamai]

Agreed. I must have missed this one when working on volumes.

[bnussman-akamai]

Looks good! Thanks for breaking out this fix into its own PR!