Parser::SourceParser#convert_encoding - fixup BOM encoding

[MSP-Greg]

Description

Ruby 3.3 may change handling of BOM encoded files. Change code to work with 3.3 and earlier versions.

Closes #1509

Note that Ruby 2.7 added IO##set_encoding_by_bom

Completed Tasks

• [x] I have read the Contributing Guide.
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [ ] I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[lsegal]

Thanks for the quick fix on #1509!