Remove links to dead developer-old.gnome.org
See https://discourse.gnome.org/t/retirement-of-developer-old-gnome-org/20991
./doc/sdk/libide.toml.in: docs_url = "https://developer-old.gnome.org/gtksourceview/stable/"
./doc/sdk/libide.toml.in: docs_url = "https://developer-old.gnome.org/libpeas/stable/"
- Should link https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/index.html or better the non-versioned and more stable https://gitlab.gnome.org/GNOME/gtksourceview
- Should link https://gnome.pages.gitlab.gnome.org/libpeas/libpeas-2/ or better the non-versioned and more stable https://gitlab.gnome.org/GNOME/libpeas