Bump MSRV from 1.61.0 to 1.70.0

[weiji14]

MSRV bump was accidentally reverted in 652721e / #257 and not reinstated.

Have verified using cargo msrv find that minimum supported version is indeed 1.70.0. Output from cargo msrv list:

  [Meta]   cargo-msrv 0.18.3

  ╭────────┬────────────────────────────────────────────────────────╮
  │ MSRV   │ Dependency                                             │
  ├────────┼────────────────────────────────────────────────────────┤
  │ 1.70.0 │ tiff, half                                             │
  ├────────┼────────────────────────────────────────────────────────┤
  │ 1.67.0 │ flate2                                                 │
  ├────────┼────────────────────────────────────────────────────────┤
  │ 1.61.0 │ jpeg-decoder                                           │
  ├────────┼────────────────────────────────────────────────────────┤
  │        │ weezl, crunchy, cfg-if, miniz_oxide, crc32fast, adler2 │
  ╰────────┴────────────────────────────────────────────────────────╯

Note: since there isn't a Cargo.lock file in this repo, I needed to pin half = { version = "=2.4.1" } in the Cargo.toml to ensure the correct MSRV of 1.70.0 was picked up, otherwise half=2.5.0 would result in an MSRV of 1.85.0.