refactor: improve execute to deduplicate key string, remove closure

[GideonBear]

What does this PR do

• Passes the ctx parameter to func in Runner::execute to remove the unnecessary closure when calling it (see main.rs line 505
• Passes the confirm_run parameter to func in Runner::execute to:
  • Remove duplication of the key/message string (see generic.rs line 1622)
  • Hide Topgrade implementation details in the run_* functions (why would they need to know to print the separator?)
  • Create warnings when the separator is not printed (because confirm_run is unused)
  • Open up the possibility for adding more functionality besides printing the separator

I haven't applied it to all calls to execute yet, to ask for approval first. Is this acceptable @SteveLauC?
Then I will (hopefully mostly automated with regex replaces) change the definitions and usages of the run_* functions.

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• If this PR introduces new user-facing messages they are translated