libphobos: Merge upstream phobos bf0d0a37c

std.array.Appender and RefAppender: use .opSlice() instead of data()

Previously, Appender.data() was used to extract a slice of the Appender's array.
Now use the [] slice operator instead.  The same goes for RefAppender.

Fixes: PR d/94455

Reviewed-on: https://github.com/dlang/phobos/pull/7450