Restore HTTP.Exceptions as a deprecating compat shim + migration note

[mathieu17g]

Implements the deprecating HTTP.Exceptions shim and migration note discussed in #1314 (context and prevalence there).

src/HTTP.jl re-adds HTTP.Exceptions as a thin deprecated submodule forwarding the four relocated types to their top-level names:

module Exceptions
    import ..HTTP
    Base.@deprecate_binding HTTPError    HTTP.HTTPError    false
    Base.@deprecate_binding StatusError  HTTP.StatusError  false
    Base.@deprecate_binding ConnectError HTTP.ConnectError false
    Base.@deprecate_binding TimeoutError HTTP.TimeoutError false
end

So catch … isa HTTP.Exceptions.StatusError resolves again, forwarding to HTTP.StatusError with a --depwarn=yes warning — same mechanism as the existing @deprecate escape escapeuri, and like it, untested.

RequestError, @try, and current_exceptions_to_string are not reinstated; the migration note covers them (RequestError has no drop-in — the underlying exception propagates and isrecoverable classifies it, #1310).

docs/src/guides/migration-1x.md gains an "Exceptions" section (Before/After for StatusError and RequestError) and a Final-Checklist line.

Closes #1314.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.17%. Comparing base (9e830d9) to head (2e44ce5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1315      +/-   ##
==========================================
- Coverage   87.32%   87.17%   -0.16%     
==========================================
  Files          29       29              
  Lines       11326    11326              
==========================================
- Hits         9890     9873      -17     
- Misses       1436     1453      +17     

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[mathieu17g]

The codecov/project -0.16% is an indirect drop in http2_server.jl's non-deterministic error/RST paths (base 10 coverage sessions vs this PR's 5; codecov/patch green; this PR's files untracked) — a re-run should re-aggregate and clear it.