Lock wms crs input to epsg-3857 (issue 600)

[qugu2427]

Description

Disable the custom wms crs input and set it to a constant value of EPSG:3857.

Currently, users are able to input a different crs into the curstom wms field. The problem is that maplibre only allows epsg-3857 (see #600 (comment)), thus inputting any other value creates an incorrect bbox which essentially ruins the whole map.

Though this is not a perfect 'fix', it does prevent these bbox errors from occuring. Additionally, most map servers will allow epsg-3857 as a source.

Issues Resolved

#600

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[junqiu-lei]

Thanks for sharing the context for maplibre supported projections, like it metioned:

MapLibre Native supports Web Mercator (EPSG:3857) as its only supported projection. This fulfills most of the web and device map needs. At the time of writing, modern map renderers such as Google Maps and Mapbox GL offers 3D globe, conic, and rectangular projections too. At the time of writing, MapLibre Native renderer component does not have an architectural separation for supporting multiple projections and coordinate reference systems.

In this case, maybe we remove crs field component on UI and use the fixed constant value of EPSG:3857 here?

[qugu2427]

@junqiu-lei Per maplibre/maplibre-gl-js#168, there is a very slow push to add support for additional crs types. My thinking was that leaving the wms crs ui field, would make it trivial to re-enable once maplibre adds support. Additionally, it would at least alert users to the fact that they can only use epsg-3857, which could save them a headache since some legacy sources do not support epsg-3857.