Deprecate use of legacy filters syntax in InMemoryDocumentStore

[silvanocerza]

The legacy filters syntax from Haystack 1 must be deprecated in favour of the new one.

As of now if the legacy syntax is used we automatically convert it internally to the new one.
We should emit a deprecation warning instead.

After that is done we can also remove some utility classes in testing used to test Document Stores.

[vblagoje]

@silvanocerza we also convert filters in MetadataRouter. To avoid tracking current and new usages of convert why don't we simply add deprecation warning to convert:

warnings.warn(
        "You are using deprecated filter syntax. Please use the new filter syntax as described "
        "in the documentation. We will attempt to convert your old filter syntax to the new one. The old syntax "
        "support will be removed in Haystack 2.5",
        DeprecationWarning,
    )

And then simply remove the relevant code handing convert invocation altogether.

[vblagoje]

Resolved with #8004