Update leaflet library to provide a11y improvements

[kimisgold]

An accessibility audit has flagged a number of challenges for screen readers:

• Poor labeling for the "close" buttons on individual marker panes.
• Map markers cannot be navigated to via keyboard.
• Screen readers do not announce the map when it comes into focus.

I think Geolocation is currently using Leaflet 1.6.0, and there have been several accessibility updates since that may address these issues.

[jimsafley]

The update-leaflet-library branch updates the Leaflet library to v1.9. Hopefully that'll address the issues.

[zerocrates]

Yes, I'd looked into this a week or so ago. The "close" button should definitely be included in this update. Not sure about the others.

[kimisgold]

I can confirm the close button issue is resolved, as well as the map markers now being navigable via keyboard. I think there are some other elements we have to do ourselves: it seems currently not possible to navigate to popup pane content once a map marker is clicked, and we need a better aria role for the map itself.

[zerocrates]

The update here is merged, along with updates to the other two leaflet-related dependencies we use.

Other things I'll note here:

• The marker clusterer did seem to have some accessibility updates, but looks like it has more (in particular aria labeling for the cluster) that were done a long time ago but haven't been released... they haven't made a release in a long time. We could pull it in manually if necessary.
• Leaflet allows/recommends setting an alt for markers which we're not currently doing, markers have an alt of "Marker" by default it looks like

[kimisgold]

I've provided the item title as an alt for the markers. (28dfc6a).

I think I'd be okay with the manual updates getting pulled in. We could always upgrade again whenever they do a formal release including any future fixes.