refactor(workspaces): +workspace/delete: rename to +workspace/kill

To be more consistent with other uses of the verb in the Emacs
ecosystem. Also done in preparation for a new +workspaces/delete command
for #7869.

Ref: #7869

modules/email/mu4e/autoload/email.el

@@ -365,7 +365,7 @@ When otherwise called, open a dired buffer and enable `dired-mu4e-attach-ctrl-c-
   ;; (prolusion-mail-hide)
   (cond
    ((and (modulep! :ui workspaces) (+workspace-exists-p +mu4e-workspace-name))
-    (+workspace/delete +mu4e-workspace-name))
+    (+workspace/kill +mu4e-workspace-name))
 
    (+mu4e--old-wconf
     (set-window-configuration +mu4e--old-wconf)

modules/email/notmuch/autoload.el

@@ -32,7 +32,7 @@
   ;; (+popup/close (get-buffer-window "*notmuch-hello*"))
   (doom-kill-matching-buffers "^\\*notmuch")
   (when (modulep! :ui workspaces)
-    (+workspace/delete +notmuch-workspace-name)))
+    (+workspace/kill +notmuch-workspace-name)))
 
 (defun +notmuch-get-sync-command ()
   "Return a shell command string to synchronize your notmuch mail with."