Upgrade OpenLayers from `v6.1.1` to `v8.2.0`

[JakobMiksch]

• only minor code adaptation was needed to perform the upgrade
• upgrade should not change anything in the UI
• change source of OpenLayers library from https://openlayers.org to https://cdn.jsdelivr.net was needed, because the OpenLayers website does not seem to provide the library anymore
• the webmap in the pgrouting example also still works after the upgrade: https://github.com/CrunchyData/pg_featureserv/blob/master/demo/routing/openlayers-pgrouting.html

[dr-jts]

Perhaps OpenLayers should just be vendored in, to avoid external dependencies?

[JakobMiksch]

Perhaps OpenLayers should just be vendored in, to avoid external dependencies?

@dr-jts True, how would you prefer to do that? I see these options:

• add a minified version of the OL library (+CSS) to the git of the project. It has the advantage, that pg_featureserv can start immediately, but checking in a dependency in to git is not always preferred
• the OL library (+CSS) needs to be downloaded when the program is built or it has do be done (semi-)manually by the person who starts a local dev setup. we might night a little script for that (or maybe go has some inbuilt functionality (?) ). The advantage is that the OL dependency stays out of the project's source code

[JakobMiksch]

@dr-jts I upgraded this PR to OpenLayers 8.2.0. I like the idea of keeping the library of OpenLayers directly in pg_feature. However, I think this should be a separate issue. What do you think, can we merge this PR first?