Tweak tryParameterless to use readapt

[som-snytt]

Readapt will autoapply if permitted and also warn on migration.

Fixes #21207

Why was the ticket worth tackling?

I started to look at tickets about overloading, so this is a first step on the journey of understanding the parts involved.

It happens that I backported tryParameterless to Scala 2.

How I fixed it

There are just two places where it adds parens.

My first attempt was more direct, since I want only a possible warning and parens:

alt.info match
case wtp: MethodType => adaptNoArgsUnappliedMethod(wtp, functionExpected = false, arity = -1)

but the tree has an overloaded type.

Why is this PR worth reviewing?

It's a one-line change, so now I'm on pace for 1 LOC/day, which is net zero.

What's the worse that could happen?

The fix is a bit indirect, so it's easy to imagine it breaking under strain.

[som-snytt]

Answers the question at #6955 (comment)

We'll see if it's the right answer.