How can I convert Scala 3.x code with mdoc CLI?

Hi, thank you for neat and useful software.

My mdoc (v2.6.1) seems not accepting Scala 3.x code from CLI.

For instance, following code does not compile:

```scala mdoc
def fib(n: Int): Int = n match
case 1 => 1
case n => n * fib(n - 1)

fib(5)
```

mdoc --in main.template.md --out main.mdoc.md
info: Compiling 1 file to main.mdoc.md
error: main.template.md:5:1: `{` expected but `case` found
case 1 => 1
^^^^
info: Compiled in 0.2s (1 error)

Can I modify Scala version from CLI?

Of course, creating entire project to compiling just one source code is such a pain...