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 https://github.com/opensearch-project/dashboards-maps/issues/600#issuecomment-2218484539), 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

https://github.com/opensearch-project/dashboards-maps/issues/600

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