[BC break] Drop support for tag filter expressions without `@` prefixes

[acoulton]

As per #407, we have always officially documented and expected that tag filter expressions would contain the @ prefix e.g. @wip&&~@slow. This was the only syntax that was ever tested.

However, we do not validate that we can parse the tag expression, and the implementation prior to 4.16.0 had coincidentally accepted filters with or without the prefix.

We have reinstated backwards compatibility with those for the time being, but we should deprecate support for invalid tag expressions and officially remove it in the next major.