Upgrade AGP to 8.1 and configure per app language (bitfireAT/davx5#338)

* Upgraded AGP

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Enabled automatic locale config generation

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Added fallback language

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Added legacy service

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Added `Accept-Language` header to custom tabs

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Nextcloud Login Flow/Google OAuth: also send language tag for default locale

---------

Signed-off-by: Arnau Mora <arnyminerz@proton.me>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>