chore: use tls lib v4 for pki integration

[gruyaume]

Description

Here we replace the tls library v3 with v4 completing the transition to use the lib v4 in Vault charms.

Checklist:

• My code follows the style guidelines of this project
• 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 unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules