Convert to Python 3.9: convert most mappings to dicts

[nolar]

An extended follow-up for:

• Deprecate Python 3.8 & upgrade to Python 3.13 in CI #1164
• Convert to Python 3.9: all the basic syntax as per pyuprade #1165

Here, minimize the usage of Mapping & MutableMapping and reduce them to the built-in generic dict.

Excluded from the conversion:

• Generic type declarations that implement the interface of Mapping/MutableMapping.
• Live views into the underlying dicts, such as Body/Spec/Meta/Labels/Annotations.
• All user-facing public API routines that accept dict-like values, such as object/label/metadata manipulation.
• All user-facing public API types that are not promised to be dicts, though can be de-facto dicts.
  • Except for the webhook HTTP headers & SSL peer info, which are now promised to be dicts.

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The Iterable / Iterator / Container / Collection / Sequence generics are NOT converted or narrowed down to their concrete types, even internally. While the conversion of mappings to dicts simplifies the codebase, the conversion of these generics would do the opposite — it will complicate the codebase. We need to know the bare minimum behaviour that we can use in a routine, and allow the callers of this routine to inject any possible containers they prefer on their side.

Such distinction is of little significance for the only two types of mappings: readonly vs. mutable — we can ignore it, we use dicts in all cases anyway. (Except for the "live views" on bodies for a bit of syntax sugar.)