Replace deprecated utf8_decode function

Problem/Motivation

Deprecated function: Function utf8_decode() is deprecated since 8.2, visit the php.net documentation for various alternatives in imagefieldintextifnone_strpos_nth() (line 171 of /home/vhfa_test/site/web/modules/contrib/imagefieldintextifnone/imagefieldintextifnone.module)

Steps to reproduce

Proposed resolution

mb_convert_encoding($utf8_string, 'ISO-8859-1', 'UTF-8'); looks like the most straightforward fix. Provides same locations as utf8_decode did for character starts testing with ç and 😵‍💫 etc in a string.

Possibly we should be reading the ISO type from the text we are getting?

Remaining tasks

User interface changes

API changes

Data model changes