Replace string concatenation with StringBuilder or string interpolation

[Shane32]

There are lots of intermediate allocations, that could be avoided.

For .NET 10 the JIT will probably elide them, but for .NET 6+ string interpolation should be used. And IIRC for older targets only some attributes need to be polyfilled, so that the compiler can emit the proper code.

Originally posted by @gfoidl in https://github.com/Shane32/QRCoder/pull/640#discussion_r2405640648