Fix "PHP Deprecated: Using ${var} in strings is deprecated, use {$var} instead"

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f64459f) 10.77% compared to head (308de84) 0.00%.

Files	Patch %	Lines
includes/LST.php	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #250 +/- ## ============================================ - Coverage 10.77% 0.00% -10.78% Complexity 1569 1569 ============================================ Files 23 23 Lines 4259 4262 +3 ============================================ - Hits 459 0 -459 - Misses 3800 4262 +462 ```

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[Universal-Omega]

Thank you!