fix: budget naming series (backport #48075)

[mergify[bot]]

Removed hard-coded naming series for Budget DocType.

Fixes: #47833

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This is an automatic backport of pull request #48075 done by Mergify.

[mergify[bot]]

Cherry-pick of c4bdf2a721de22fd596bc2a378f02727d7aeb2cc has failed:

On branch mergify/bp/version-15-hotfix/pr-48075
Your branch is up to date with 'origin/version-15-hotfix'.

You are currently cherry-picking commit c4bdf2a721.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/accounts/doctype/budget/budget.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/accounts/doctype/budget/budget.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally