fix(signup): bump gleam_httpc to >= 4.2.0 for result.try

gleam_httpc 4.1.1 still calls result.then, which was removed from
gleam_stdlib 0.71.0. Bumping the lower bound forces the resolver to
pick 4.2.x or later, which uses result.try. The < 5.0.0 cap is also
widened to < 6.0.0; the public API is unchanged between 4.2 and 5.0.

Long term: committing a manifest.toml lockfile would stop transitive
deps from drifting on every fresh build.

signup/gleam.toml

@@ -5,7 +5,7 @@ target = "erlang"
 [dependencies]
 gleam_stdlib = ">= 0.59.0 and < 2.0.0"
 gleam_http = ">= 4.0.0 and < 5.0.0"
-gleam_httpc = ">= 4.0.0 and < 5.0.0"
+gleam_httpc = ">= 4.2.0 and < 6.0.0"
 gleam_json = ">= 3.0.0 and < 4.0.0"
 gleam_erlang = ">= 1.0.0 and < 2.0.0"
 envoy = ">= 1.0.0 and < 2.0.0"