[Serializer] Add support for ignoring comments while XML encoding

[javiereguiluz]

See symfony/symfony#28289.

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[carsonbot]

Just a quick reminder to make a comment on this. If I don't hear anything I'll close this.

[wouterj]

Fixed by #14384