fix SB/flux scale factor

[kecnry]

Description

This pull request updates the scale-factor used for converting a spectrum between surface-brightness and flux units to use the wavelength-dependent aperture area (array) instead of the cone's scale factor at the current slice (float).

This should be rebased on top of main after #2859 is merged. See https://github.com/spacetelescope/jdaviz/pull/2859#discussion_r1594009067 for more details.

Change log entry

• [ ] Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts, list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• [ ] Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• [ ] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [ ] Do the proposed changes follow the STScI Style Guides?
• [ ] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Did the CI pass? If not, are the failures related?
• [ ] Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• [ ] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?