fix: do not add scala.util.Properties as the Main-Class in the Manifest

[hamzaremmal]

When checking the MANIFEST.MF file of the stdlib 3.8.0-RC1, I observed that we list Main-Class: scala.util.Properties

Manifest-Version: 1.0
Specification-Title: scala-library-bootstrapped
Specification-Version: 3.8.0-RC1
Specification-Vendor: LAMP/EPFL
Implementation-Title: scala-library-bootstrapped
Implementation-Version: 3.8.0-RC1
Implementation-Vendor: LAMP/EPFL
Implementation-Vendor-Id: org.scala-lang
Implementation-URL: https://scala-lang.org/
Main-Class: scala.util.Properties

In this PR, we make sure to not list any Main-Class for the stdlib since it is a library, not an application.

[som-snytt]

The explanatory comment is

// provide a main method so version info can be obtained by running this

[hamzaremmal]

They should be able to run that method by specifying it manually, not automatically.
Even in Scala 2.13, if we have:

java -jar scala-library-2.13.18.jar 
no main manifest attribute, in scala-library-2.13.18.jar

I don't see why we should have Scala 3 behave differently.

java -jar scala-library-3.8.0-RC2.jar 
Scala library version 3.8.0-RC2 -- Copyright 2002-2025, LAMP/EPFL

The way I see things is: this is a library, not an application. It doesn't make a lot of sense to me to have an entry point available when we try to execute the JAR.