Components.WebView: Fix CI failure building with WebKitGTK >= 2.31

The `webkit_settings_set_enable_plugins` method has been deprecated in
2.31, and calling it emits a warning which trips up the test suite.

Cherry picked from !624 commit 28921890
3 jobs for mjog/webkit_plugin_warning-3_38 in 9 minutes and 58 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1110988
fedora

00:07:00

passed #1110989
ubuntu

00:03:42

 
  Test
failed #1110990
flatpak
flatpak

00:02:58

 
Name Stage Failure
failed
flatpak Test
Uploading artifacts for failed job
Uploading artifacts...
WARNING: geary-dev.flatpak: no matching files
WARNING: repo.tar: no matching files
WARNING: .flatpak-builder/build/geary/_flatpak_build/meson-logs/meson-log.txt: no matching files
WARNING: .flatpak-builder/build/geary/_flatpak_build/meson-logs/testlog.txt: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1