fix(ui): use background delete to match k8s terminology

[jsoref]

In #15577 I ran into a problem where I couldn't figure out how the delete options ArgoCD presented related to the Kubernetes deletion strategies:

It's problematic to not use the same terms that the underlying system uses if the underlying system's documentation/behavior is something a user is likely to encounter. (If they're unlikely to ever encounter it, then introducing alternate terminology is fine, but that doesn't seem likely for ArgoCD & Kubernetes.)

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
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• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7484f1d) 49.49% compared to head (bfabd54) 49.49%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #15579   +/-   ##
=======================================
  Coverage   49.49%   49.49%           
=======================================
  Files         270      270           
  Lines       47489    47489           
=======================================
  Hits        23506    23506           
  Misses      21672    21672           
  Partials     2311     2311           

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