Use unknown instead of any in *MapTo operators

[krokosik]

Currently operators like mapTo, mergeMapTo and so on, have a type of OperatorFunction<any, T>, which does not cause any errors on its own, but introduces an unsafe any argument, which is picked up by this ESLint rule. Changing it to unknown, is a rather cosmetic fix, but It fixes the problem and is more in line with how TypeScript should be used.

[kwonoj]

#6399

Core team have long term planning to deprecate *To variant eventually. If current eslint rule is blocking, recommendation is use map(() => ) instead.

[Wyzard256]

This was recently resolved by #6738.