Bump MSRV to 1.88.0 (apache#18403)

## Description

This PR bumps the Minimum Supported Rust Version (MSRV) from 1.87.0 to
1.88.0 per our MSRV policy.

Rust 1.91.0 was released today:
https://blog.rust-lang.org/2025/10/30/Rust-1.91.0/

Per our MSRV policy we can now upgrade from 1.87.0 to 1.88.0. The policy
states that DataFusion supports the last 4 stable Rust minor versions
released and any such versions released within the last 4 months.

## Changes

- Updated `rust-version` in `Cargo.toml` from `1.87.0` to `1.88.0`
- Added upgrade note in `docs/source/library-user-guide/upgrading.md`

## Testing

- ✅ Verified the code compiles with Rust 1.90.0 (newer than MSRV)
- ✅ All workspace crates compile successfully

## Related

Closes apache#18395

## Notes

This change enables the use of Rust 1.88.0 features, including let
chains, which are now available for use in the codebase.

Author: harshasiddartha

Cargo.toml

@@ -77,7 +77,7 @@ license = "Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/apache/datafusion"
 # Define Minimum Supported Rust Version (MSRV)
-rust-version = "1.87.0"
+rust-version = "1.88.0"
 # Define DataFusion version
 version = "50.3.0"
 

docs/source/library-user-guide/upgrading.md

@@ -34,11 +34,11 @@ to version `57.0.0`, including several dependent crates such as `prost`,
 `tonic`, `pyo3`, and `substrait`. . See the [release
 notes](https://github.com/apache/arrow-rs/releases/tag/57.0.0) for more details.
 
-### `MSRV` updated to 1.87.0
+### `MSRV` updated to 1.88.0
 
-The Minimum Supported Rust Version (MSRV) has been updated to [`1.87.0`].
+The Minimum Supported Rust Version (MSRV) has been updated to [`1.88.0`].
 
-[`1.87.0`]: https://releases.rs/docs/1.87.0/
+[`1.88.0`]: https://releases.rs/docs/1.88.0/
 
 ### `FunctionRegistry` exposes two additional methods