Minor: Rephrase MSRV policy to be more explanatory

[comphead]

Which issue does this PR close?

Looks like the MSRV policy on hotfixes can be slightly reworded and improved

Closes #.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[findepi]

What about slightly different approach to the wording:

## Rust Version Compatibility Policy

The Rust toolchain releases are tracked at [Rust Versions](https://releases.rs) and follow
[semantic versioning](https://semver.org/). A Rust toolchain release can be identified
by a version string like `1.80.0`, or more generally `major.minor.patch`.
A Rust toolchain of given version can be officially released or in preparation,
as stated on the Rust Versions page.

DataFusion's Minimum Required Stable Rust Version (MSRV) policy is the following:

- the four latest stable Rust minor releases are supported.
  For example if the stable releases are `1.81.x`, `1.80.x`, `1.79.x`, `1.78.x`, `1.77.x`,
  then `1.81.x`, `1.80.x`, `1.79.x`, `1.78.x` are the four latest minor releases and are supported.
- the latest stable release as of four months ago is supported.
  For example if the current date is `2024-12-01`, then the latest stable release as of `2024-08-01`
  is supported.

Rust occasionally releases patch releases for a stable minor version.
Only the latest patch release for a stable minor version is supported.

We enforce this policy using a [MSRV CI Check](https://github.com/search?q=repo%3Aapache%2Fdatafusion+rust-version+language%3ATOML+path%3A%2F%5ECargo.toml%2F&type=code)

[comphead]

What about slightly different approach to the wording:

## Rust Version Compatibility Policy

The Rust toolchain releases are tracked at [Rust Versions](https://releases.rs) and follow
[semantic versioning](https://semver.org/). A Rust toolchain release can be identified
by a version string like `1.80.0`, or more generally `major.minor.patch`.
A Rust toolchain of given version can be officially released or in preparation,
as stated on the Rust Versions page.

DataFusion's Minimum Required Stable Rust Version (MSRV) policy is the following:

- the four latest stable Rust minor releases are supported.
  For example if the stable releases are `1.81.x`, `1.80.x`, `1.79.x`, `1.78.x`, `1.77.x`,
  then `1.81.x`, `1.80.x`, `1.79.x`, `1.78.x` are the four latest minor releases and are supported.
- the latest stable release as of four months ago is supported.
  For example if the current date is `2024-12-01`, then the latest stable release as of `2024-08-01`
  is supported.

Rust occasionally releases patch releases for a stable minor version.
Only the latest patch release for a stable minor version is supported.

We enforce this policy using a [MSRV CI Check](https://github.com/search?q=repo%3Aapache%2Fdatafusion+rust-version+language%3ATOML+path%3A%2F%5ECargo.toml%2F&type=code)

I like part about semver so its removes ambiguity about major and minor versions. For the policy itself I feel it can be easier to read... let me reword it

[comphead]

@alamb @findepi can I get the review on this?

[alamb]

Thanks @comphead - I think this PR is better than what is on main. I have a suggestion to perhaps make it clearer.