clean up dead links, fix typo

[CicadaCinema]

I fixed any dead links beginning https://matrix.org/speculator and some issues I found along the way.

https://web.archive.org/web/20190329152312/https://matrix.org/speculator/spec/HEAD/client_server/unstable.html#user-interactive-authentication-api
is now found at
https://spec.matrix.org/v1.7/client-server-api/#user-interactive-authentication-api

https://web.archive.org/web/20170620093435/https://matrix.org/speculator/spec/HEAD/client_server/unstable.html#post-matrix-client-unstable-register
is now found at
https://spec.matrix.org/v1.7/client-server-api/#post_matrixclientv3register

https://github.com/matrix-org/matrix-spec/blob/2a8d64fef7a40717ef9f5748ee0551b2117be037/specification/intro.rst?plain=1#L443
is now found at
https://spec.matrix.org/v1.7/appendices/#user-identifiers

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.15 ⚠️

Comparison is base (2ee03fd) 65.56% compared to head (6a55f0b) 65.41%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3130      +/-   ##
==========================================
- Coverage   65.56%   65.41%   -0.15%     
==========================================
  Files         504      504              
  Lines       56055    56073      +18     
==========================================
- Hits        36751    36680      -71     
- Misses      15517    15583      +66     
- Partials     3787     3810      +23     

Flag	Coverage Δ
unittests	49.76% <0.00%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
clientapi/routing/register.go	72.74% <ø> (ø)
internal/validate.go	100.00% <ø> (ø)
setup/config/config_clientapi.go	75.00% <0.00%> (+1.31%)	⬆️

... and 21 files with indirect coverage changes

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