ENH Don't use keyword "self"

[GuySartorelli]

Fixes https://github.com/silverstripe/silverstripe-assets/actions/runs/10088694351/job/27894872547

 Error: Can't use keyword 'self'. Use 'ImageShortcodeProvider' instead.
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  Line   Shortcodes/ImageShortcodeProvider.php                            
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  139    Can't use keyword 'self'. Use 'ImageShortcodeProvider' instead.  
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Issue

• Broken builds and merge-ups .github#287

[emteknetnz]

Should target 2.2?

[GuySartorelli]

This code doesn't exist in 2.2