[GTK] Only implement GtkAccessible in recent versions

[GeorgesStavracas]

4aa6d21

[GTK] Only implement GtkAccessible in recent versions
https://bugs.webkit.org/show_bug.cgi?id=227528

Reviewed by Adrian Perez de Castro.

GtkAccessible is only publicly exposed as an externally implementable
interface since GTK 4.10, but Ubuntu 22.04 ships with GTK4 4.6. This
breaks the build.

Conditionally implement this interface based on the availability of
the GTK_ACCESSIBILITY_ATSPI define.

* Source/WebKit/UIProcess/API/gtk/WebKitWebViewBase.cpp:
(webkitWebViewBaseAccessibleGetFirstAccessibleChild):

Canonical link: https://commits.webkit.org/274284@main

ee5991a

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	🧪 api-mac	✅ 🧪 api-wpe
	🧪 ios-wk2-wpt		✅ 🛠 gtk
	✅ 🧪 api-ios	🧪 mac-wk2	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
	🛠 watch
✅ 🛠 🧪 unsafe-merge	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash ee5991a)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	🧪 api-mac	✅ 🧪 api-wpe
	🧪 ios-wk2-wpt		✅ 🛠 gtk
	✅ 🧪 api-ios	🧪 mac-wk2	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
	🛠 watch
✅ 🛠 🧪 unsafe-merge	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 274284@main (4aa6d21): https://commits.webkit.org/274284@main

Reviewed commits have been landed. Closing PR #24066 and removing active labels.