Remove direct use of Keytar and use new `SecretStorage` API

[bwateratmsft]

In 1.53.0, VSCode added a new SecretStorage API. We currently use keytar directly. We should remove our usage of keytar in favor of the SecretStorage API.

[bwateratmsft]

Note: this same change ought to be made in the extensibility repo.

[bwateratmsft]

This is now released in Docker extension version 1.11.0. Please note, due to this change users will need to reconnect to their registries (i.e. reenter their credentials).