Avoid OrderedIterator when comparing OpenApiSchema.Required equality

[captainsafia]

Contributes towards #56829.

Required properties in an OpenAPI schema represented in an ISet. Previously, to support order-invariant comparisons we were using .Order() and .SequenceEquals().

It turns out .SetEquals() exists and is order invariant so we can use that directly instead of constructing an OrderedIterator via the Order call.

The SetEquals method ignores duplicate entries and the order of elements in the other parameter.
-- SetEquals API Remarks

Results from a local benchmark run are below:

BenchmarkDotNet v0.13.12, macOS Sonoma 14.6.1 (23G93) [Darwin 23.6.0]
Apple M2 Max, 1 CPU, 12 logical and 12 physical cores
.NET SDK 9.0.100-preview.7.24371.4
[Host] : .NET 9.0.0 (9.0.24.41005), Arm64 RyuJIT AdvSIMD
Job-JUVRYQ : .NET 9.0.0 (9.0.24.36618), Arm64 RyuJIT AdvSIMD

Server=True Toolchain=.NET Core 9.0 RunStrategy=Throughput

Before

Method	EndpointCount	Mean	Error	StdDev	Op/s	Gen0	Gen1	Allocated
GenerateDocument	10	316.7 μs	6.20 μs	6.09 μs	3,157.61	3.9063	-	586.53 KB
GenerateDocument	100	3,066.2 μs	58.45 μs	51.82 μs	326.14	31.2500	-	5336.09 KB
GenerateDocument	1000	32,198.7 μs	387.17 μs	343.22 μs	31.06	333.3333	166.6667	52828.55 KB

After

Server=True Toolchain=.NET Core 9.0 RunStrategy=Throughput

Method	EndpointCount	Mean	Error	StdDev	Op/s	Gen0	Gen1	Allocated
GenerateDocument	10	300.1 μs	0.73 μs	0.65 μs	3,331.88	3.9063	-	538.06 KB
GenerateDocument	100	2,776.9 μs	22.87 μs	21.39 μs	360.11	31.2500	15.6250	4891 KB
GenerateDocument	1000	30,593.4 μs	595.03 μs	636.68 μs	32.69	166.6667	-	48415.86 KB