MSRV CI check is failing on main

[alamb]

Describe the bug
The MSRV CI check is failing

Example
https://github.com/apache/arrow-rs/actions/runs/13864941361/job/38801865299

{"type":"subcommand_result","subcommand_id":"verify","result":{"toolchain":{"version":"1.70.0","target":"x86_64-unknown-linux-gnu","components":[]},"is_compatible":false,"error":" Updating crates.io index\n Downloading crates ...\n Downloaded once_cell v1.21.1\n Downloaded chrono v0.4.40\n Downloaded libc v0.2.171\n Downloaded half v2.5.0\n Downloaded bytes v1.10.1\nerror: package half v2.5.0 cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.70.0\nEither upgrade to rustc 1.81 or newer, or use\ncargo update -p half@2.5.0 --precise ver\nwhere ver is the latest version of half supporting rustc 1.70.0\n"}}

https://crates.io/crates/half/2.5.0 was released 18 hours ago about when the CI started failing

To Reproduce

Expected behavior
CI should pass

Additional context
We really need to sort out

• Adopt a MSRV policy #181