transformPartitionBoundValue went out of its way to do the wrong
thing: there is no reason to complain about a non-matching COLLATE
clause in a partition boundary expression.  We're coercing the
bound expression to the target column type as though by an
implicit assignment, and the rules for implicit assignment say
that collations can be implicitly converted.

What we *do* need to do, and the code is not doing, is apply
assign_expr_collations() to the bound expression.  While this is
merely a definition disagreement, that is a bug that needs to be
back-patched, so I'll commit it separately.

Discussion: https://postgr.es/m/CAJV4CdrZ5mKuaEsRSbLf2URQ3h6iMtKD=hik8MaF5WwdmC9uZw@mail.gmail.com