Raise maxVertexBuffers default to 16

[kainino0x]

Currently maxVertexBuffers defaults to 8, but this is a regression compared with WebGL 2.0. All of the backends support 16, except...

In WebGPU implementations on Metal which are not using Metal's Argument Buffers, maxVertexBuffers contends with maxStorageBuffersPerShaderStage and maxUniformBuffersPerShaderStage for the Metal limit Maximum number of entries in the buffer argument table, per graphics or kernel function (#693 (comment)).

With argument buffers, this is no longer a problem, because we added the combined limit maxBindGroupsPlusVertexBuffers to take care of this (#2749). We just didn't add a limit for non-argument-buffers cases because that's backward-looking.

Per #1069 (comment) we no longer officially support Metal versions without argument buffers. However we'll want to hold off on raising this base limit until implementations are actually migrated to using argument buffers, so IMO this is no earlier than Milestone 2.