Bump MSRV to 1.70.0

The intention is to take advantage of `std::io::IsTerminal` that landed
in 1.70.0, both in `bat` and its dependencies (`clap`, `grep-cli`).

This will fix #2570 as well - `grep-cli` 0.1.9 has a patch for that.

Signed-off-by: mataha <mataha@users.noreply.github.com>

src/bin/bat/app.rs

@@ -1,9 +1,8 @@
 use std::collections::HashSet;
 use std::env;
+use std::io::IsTerminal;
 use std::path::{Path, PathBuf};
 
-use is_terminal::IsTerminal;
-
 use crate::{
     clap_app,
     config::{get_args_from_config_file, get_args_from_env_opts_var, get_args_from_env_vars},