main/libsecret: upgrade to 0.21.0
Changelog
0.21.0
* Unlock the keyring before getting secret attributes [!105]
* file-backend: avoid deadlock when portal op is canceled [!118, !120]
* Properly chain-up GTasks around GDBusProxy::init_async [!106]
* Fix nullable in secret_collection_for_alias_* [!110]
* Fix finish function for searchv [!112]
* secret-paths: Fix a little memory leak [!109]
* Stop using GSlice [!116]
* docs: port to gi-docgen [!101]
* docs: fix example in usage docs [!117]
* Several CI fixes [!111, !114]
* Updated translations