Trim trailing comma and space in the "to" string

Problem/Motivation

We have many ECA email actions that builds recipient string for easy_email, by looping each email like this: [email]; [emails] where emails was initialized as an empty string in ECA. This results in a tailing semicolon and space. While I manage to change all semicolons to commas (thanks to the fix for issue 3254085 we can use comma separated emails reliably), it's cumbersome to add a tamper step for each email recipient string.

Steps to reproduce

Use the above ECA approach to build a to string like "a@example.com, b@example.com, ". Then the system will warn that " " is not a valid email.

Proposed resolution

While is is not an issue of the Symfony Mailer module, trimming the trailing space and comma is helpful for the above use case.

Let me create an issue fork for this one. I would appreciate it if you could consider using it.