Bump Polyfill from 7.14.0 to 7.15.0 in /src (#257)

Author: dependabot[bot]

src/Directory.Packages.props

@@ -9,7 +9,7 @@
     <PackageVersion Include="NUnit" Version="4.3.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="PdfPig" Version="0.1.9" />
-    <PackageVersion Include="Polyfill" Version="7.14.0" />
+    <PackageVersion Include="Polyfill" Version="7.15.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.147" />
     <PackageVersion Include="Verify" Version="28.9.0" />
     <PackageVersion Include="Verify.DiffPlex" Version="3.1.2" />