emulators/flycast: Update 2.5 => 2.6

Changelog:
https://github.com/flyinghead/flycast/releases/tag/v2.6

• Update git submodules to latest release versions and reorder them into alphabetical sequence.
• Remove set-include in vulkan_context.cpp patch because it's now in upstream.
• Make slight simplification in post-install-DOCS-on.

PR: 292421