Make SIP-72 Dedented Multiline String Literals a preview feature

[bracevac]

SIP-72 was accepted by the SIP committee on August 28 2026 and should transfer from experimental to preview in the next minor release.

[bracevac]

While this was accepted after the 3.10 release cutoff, could we still have it in there? @Gedochao

[Gedochao]

@bracevac As a rule, we don't introduce new features in the release candidate phase... (not beyond RC1) but given that it's just a transition from experimental to preview, I think we could still get away with it. Would have to go in RC2, however.
cc @WojciechMazur

[Gedochao]

As per last Scala Core, this will be made -preview in 3.11