raise the MSRV to 1.88

[sunshowers]

dropshot_endpoint already uses a let chain (stabilized in Rust 1.88), so the declared MSRV of 1.85 no longer builds:

error[E0658]: `let` expressions in this position are unstable
   --> dropshot_endpoint/src/api_trait.rs:235:20

Specify an MSRV of 1.88 and fix CI to check for it. In upcoming commits we're going to add more let chains.

After #1695 and this PR, the CI test matrix is:

• all tests except trybuild ones on MSRV and stable, and across Linux/macOS/Windows
• trybuild tests on pinned toolchain on Linux

[sunshowers]

Revision history

Version	Changes from previous version	Submitted commit
4 (current)	78b4c7ff..e0e2280e	e0e2280e
3	c87c1ee5..78b4c7ff	78b4c7ff
2	01b7b044..c87c1ee5	c87c1ee5
1	Initial version	01b7b044

_{Generated by jj-stack from this pull request's force-push history.}