watchdog at info level or use rules debug

Problem/Motivation

At https://git.drupalcode.org/project/variable_email/-/blob/1392e192c0c30be... there is a call to watchdog() which omits the $severity argument.

This means each email sent creates a NOTICE level log entry, which I think is "noisier" than it should be. A site using tools to identify PHP E_NOTICE reports will also see reports of each email. If INFO or DEBUG level was used, this would sit at an appropriate level: informational, not notifiable.

We could also use rules debug, since this occurs in a Rules action.

Steps to reproduce

- Send an email via Variable Email action
- See NOTICE in Drupal logs

Proposed resolution

- Add WATCHDOG_INFO to the watchdog() call, or
- Switch to Rules debugging

Remaining tasks

- Patch

User interface changes

API changes

Data model changes

Log messages will be at a decreased level.