build: increase Minimum Supported Rust Version (MSRV) to 1.60

[intgr]

Needed to update tempfile dev-dependency.

It seems older Rust versions validate dev-dependencies with registry even if build does not need dev-dependencies. And that fails with error:

0.749     Updating crates.io index
33.93 error: failed to select a version for the requirement `rustix = "^0.38.21"`
33.93 candidate versions found which didn't match: 0.38.9, 0.38.8, 0.38.7, ...
33.93 location searched: crates.io index
33.93 required by package `tempfile v3.8.1`
33.93     ... which satisfies dependency `tempfile = "^3.8.1"` of package `posix-acl v1.1.0 (/root/build)`