fix: use "unknown" type for operators that ignore input values

[Wyzard256]

Description:

After upgrading an application's typescript-eslint dependency from 4.30 to 5.9, I found that the "@typescript-eslint/no-unsafe-argument" rule was flagging my use of the RxJS mapTo and ignoreElements operators, because they return functions that operate on observables of any. Since the operator functions ignore the values from the source observable, they don't need the dynamic typing that any provides for accessing the values. Using unknown instead makes it clear (to both eslint and humans) that the operator function doesn't have any expectations about the type of observable it's given.

This PR changes OperatorFunction<any, ...> to OperatorFunction<unknown, ...> in the return type of ignoreElements and the mapTo family of operators. No implementation changes are needed in the functions, since the associated values are never used anyway. This shouldn't be a breaking change: unknown is the same as any from a caller's perspective (no restrictions on what can be passed in), and OperatorFunction<unknown, ...> is assignable to OperatorFunction<any, ...> so anything that explicitly expects the latter is still OK.

[Wyzard256]

Looks like the CI build failed because it's an API change? I'm not familiar with api_guardian typical usage and it's not mentioned in the contributor guide, so I don't know whether there's an action I should take (like editing api_guard/dist/types/index.d.ts as part of this PR), or if it's something that a maintainer will handle.

(Edit: looking at other PRs, I see that they include modifications to the api_guard file for intentional API changes, so I'll do the same.)