nixos/matrix-synapse: fix type of `settings.url_preview_url_blacklist`

Actually, it's supposed to be `listOf (attrsOf str)` because each
list-item can match against multiple properties from `urlsplit`[1]. In
fact, `listOf str` breaks URL previews at runtime:

    Sep 14 15:03:47 soost synapse[1100355]: synapse.http.server: [GET-116] Failed handle request via 'PreviewUrlResource': <XForwardedForRequest at 0x7f691bd5f730 method='GET' uri='/_matrix/media/r0/preview_url?url=<redacted>' clientproto='HTTP/1.1' site='8448'>
                                            Traceback (most recent call last):
                                              [...]
                                              File "/nix/store/xk5yksbw09p6qwk0maq2cb2in3z6f4gn-matrix-synapse-1.91.2/lib/python3.10/site-packages/synapse/media/url_previewer.py", line 398, in _is_url_blocked
                                                for attrib, pattern in entry.items():
                                            AttributeError: 'str' object has no attribute 'items'

To make sure that people aren't confused when upgrading their configs, I
decided to work with `types.coercedTo` to "pretend" accepting the old
type signature, but then throwing an error explaining what to do (and
rejecting the broken configuration).

[1] https://docs.python.org/3/library/urllib.parse.html#urllib.parse.urlsplit

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