update MSRV to 1.57

[michaelkirk]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGELOG.md if knowledge of this change could be valuable to users.

Fixes build error: https://github.com/georust/gpx/runs/7813032321?check_suite_focus=true

error: package time v0.3.13 cannot be built because it requires rustc 1.57.0 or newer, while the currently active rustc version is 1.56.1

[lnicola]

# Two most recent releases - we omit older ones for expedient CI

Yeah, about that.. :sweat_smile:

bors r+

[bors[bot]]

Build succeeded:

• ci result