- bumps the MSRV on CI to 1.75.
- updates the `Cargo.toml` rust-version to 1.75.
- removes the guideline to pin `rustls` version, as previously required
by old 1.63 MSRV.
- update `README.md` badges to `1.75` MSRV.
- update `clippy.toml` to `1.75` MSRV.
`rustls` versions 0.23.18 and 0.23.19 contains fix for vulnerability
RUSTSEC-2024-0399. However, 0.23.18 bumps MSRV to 1.71. 0.23.19 reverts
MSRV back to 1.63.
We enforce min `rustls` version to 0.23.19 to make it easier to compile
on MSRV and ensure we include the RUSTSEC-2024-0399 fix.
Note that in CI, I decided to pin `rustls` dependency to 0.23.19
explicitly. This is because in future versions of `rustls`, the MSRV
will be changed to 1.71.
