Adds a new top-level HttpStatus enum covering the 19 status codes that s 0
FeignException recognises and uses it to replace the literal 400,
401, 403, …, 504 magic numbers in the fourteen FeignClientException /
FeignServerException subclasses and in the clientErrorStatus /
serverErrorStatus switch statements. Also replaces the 204 / 205
literals in Logger.logAndRebufferResponse. Switches now switch on
HttpStatus constants, so the status-to-exception mapping reads off the
constant names directly.

Four follow-ups in the same files:

• Logger.Level gets an atLeast(Level) helper. The four
  logLevel.ordinal() >= Level.X.ordinal() comparisons in logRequest,
  logAndRebufferResponse, and logIOException become
  logLevel.atLeast(Level.X). logAndRebufferResponse also loses its
  header loop and body rebuffering into logResponseHeaders(...) and
  rebufferBody(...) private helpers.
• Response.Builder's seven fields are now private. The
  Response(Builder) constructor in the same file routes through
  package-private getters, restoring the immutability contract the ╮
  class javadoc already advertises.
• Request.Options.getMethodOptions / setMethodOptions previously
  had a check-then-act race on the ConcurrentHashMap<String, Map<String, Options>> and used a plain HashMap for the inner map. They now use
  computeIfAbsent and a ConcurrentHashMap inner map.
  AsynchronousMethodHandler.CancellableFuture.inner is now an
  AtomicReference<CompletableFuture<T>> so retry callbacks no longer
  race with cancel(). Request.Body.data becomes final; the no-arg
  Body() constructor delegates to this(null). ╮
• The deprecated RequestTemplate.resolve(Map, Map) overload
  (alreadyEncoded was ignored) is removed. All in-repo callers use
  the single-arg resolve(Map).

No public-API additions beyond HttpStatus and Logger.Level#atLeast.
japicmp is unchanged.

$ ./mvnw -pl core test ╮
... Tests run: 634, Failures: 0, Errors: 0, Skipped: 3