Update understrap_kses_title() to not strip WPML flags

[IanDelMar]

Description

Alternative to #1850

Motivation and Context

See #1850

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• composer cs:check has passed locally.
• composer lint:php has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Further comments

• @bacoords what do you think?
• I cannot replicate the WPML flag issue and therefor cannot test whether this PR resolves the issue.

[bacoords]

• It does solve the issue of WPML by allowing SVGs in the menu text
• This still leaves the Polylang issue where Base64 encoded images in menu tags are not enabled Compatibility with WPML - Flags do not show when language switcher in menu #1468

I'll look into this more and compare the two PRs

[IanDelMar]

I got it wrong. I was thinking that WPML flags not Polylang flags have a data uri.