server: add "LAYER" to allowed parameters for GetLegendGraphic

[JakobMiksch]

• for the WMS request GetLegendGraphic it is allowed to also send the parameter LAYER and not only LAYERS (with S) as described previously, this PR fixes this
• this is important because other geospatial software like GeoServer, MapServer and OpenLayers rely on the paramter LAYER
• Backport to LTR documentation is requested

[DelazJ]

@JakobMiksch Do you know if the other requests of wms service also use LAYER or is it GetLegendGraphic only? I'm asking because the section we link to concerns all the request types.

[JakobMiksch]

@JakobMiksch Do you know if the other requests of wms service also use LAYER or is it GetLegendGraphic only? I'm asking because the section we link to concerns all the request types.

I will check and get back to you

[JakobMiksch]

@JakobMiksch Do you know if the other requests of wms service also use LAYER or is it GetLegendGraphic only? I'm asking because the section we link to concerns all the request types.

@DelazJ Here is what I could find out:

• for DescribeLayer only LAYERS (plural) is possible
• for GetStyle both LAYER (singular) and LAYERS (plural) is possible
• for GetMap both LAYER (singular) and LAYERS (plural) is possible
• for GetFeatureInfo both LAYER (singular) and LAYERS (plural) is possible
• for GetPrint I could not check

[DelazJ]

See #8584 for a more "complete" PR.