Update libgit2-sys, resolve dependabot vulnerability

[who-biz]

Addresses the following security advisory from GitHub's dependabot:
https://github.com/EpicCash/epic/security/dependabot/128

This fix only affects build-time dependencies. Version update was required for built crate, which pulled in the vulnerable libgit2-sys via its own dependencies.

The rest of the changes consist of: 1.) adjusting usage of the built crate for newer version, 2.) Adding git and cargo-lock as features to built crate (these are no longer default features), and 3.) Removal of deprecated function calls from codebase to accommodate.

Tested fully, and built_info fields such as built_info::GIT_VERSION, PKG_VERSION, DEPENDENCIES etc are all generating correct values.