Linter: Replace GIT_REV with GIT_BRANCH

[cmichi]

When the linter is run for a contract, we fetch the lints at this commit from the ink! repo:

    /// Git repository with ink_linting libraries
    pub const GIT_URL: &str = "https://github.com/paritytech/ink/";
    /// Git revision number of the linting crate
    pub const GIT_REV: &str = "b6880dd9384e09ec4e7ad65453cd844113e8a316";

(source)

I think what we should do instead:

• Read the MAJOR ink version from the contract's Cargo.toml
• Take that number (e.g. 5) and form it into v5.x.x
• Fetch the lints from the ink repository at the branch v5.x.x, instead of at a fixed GIT_REV

Right now, the linter uses the lints from GIT_REV for a contract of an arbitrary ink version. It's more future proof to have it fetch the lints for that particular ink! version. Also, we won't have to update cargo-contract any time we release new lints for an ink! version.