flatpak: Upgrade runtime to 26.08

We're currently on 25.08 (upgraded in https://github.com/wezterm/wezterm/commit/fa0a1da0f93f2ce094d06a91747dad83e362dde0), we might as well move up to 26.08 before next release!

Related:

• https://github.com/flathub/org.wezfurlong.wezterm/issues/24
• https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/tags

Internal documentation: https://github.com/wezterm/wezterm/blob/main/docs-internal/flatpak-distribution.md