chore: update license to BUSL-1.1

[gruyaume]

Description

The upstream Vault is now licensed under BUSL-1.1, and here we make the necessary changes for the rock to be distributed under this same license.

Fixes #88

Rationale

Although the BCL is not a FOSS license, it works like one in two important ways: (1) it allows for derivative works (our snap), and (2) it applies to those derivative works.

The snap is therefore under the same BCL as the software it encapsulates BCL'd software.

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

[gruyaume]

The build is failing because of a known issue in Rockcraft:

• List of supported SPDX licenses is outdated  rockcraft#372