Document the MSRV of the stable channel as 1.63

Currently this crate maintains the `main` branch (future 1.0) alongside
`libc-0.2` (current stable release). PRs are made against `main` then
cherry picked as applicable to `libc-0.2`.

Usually this flow works okay, but there is a substantial difference in
minimum supported versions: libc-0.2 is tested down to 1.19 and main is
tested with 1.63. This means that supported features differ quite a bit
and as a result, cherry picks get conflict-heavy (e.g. `repr(align)`,
`union`, and `const fn` cannot be used on `libc-0.2`).

In order to make it easier to keep these branches in sync and get us
close to a 1.0 release, raise the MSRV on `libc-0.2` to 1.63. This means
both branches can run the exact same tests.

This still does not establish a MSRV policy, which has been discussed at
great length in [1]. For the purpose of unsticking us this selects 1.63
as the MSRV, which is the version currently available on Debian stable
(a commonly requested reference point in [1], and about the oldest
specific version mentioned).

This is a documentation-only change to keep things simple, cleanup can
follow. Further increases and official policy are not precluded.

An attempt to raise the MSRV to 1.57 in 2022 was approved at one point
[2], but never merged due to various failures. Making this a
documentation-only change hopes to avoid this problem.

I brought up a 0.3 release to increase versions in [3], but consensus
there was that we should be able to increase the MSRV in the existing
0.2 release without a semver-breaking change.

Link: rust-lang/libs-team#72 [1]
Link: rust-lang#2845 [2]
Link: rust-lang/libs-team#463 [3]

README.md

@@ -55,21 +55,16 @@ libc = "0.2"
 
 ## Rust version support
 
-The minimum supported Rust toolchain version is currently **Rust 1.13.0**. (libc
-does not currently have any policy regarding changes to the minimum supported
-Rust version; such policy is a work in progress.) APIs requiring newer Rust
-features are only available on newer Rust toolchains:
-
-| Feature              | Version |
-| -------------------- | ------- |
-| `union`              | 1.19.0  |
-| `const mem::size_of` | 1.24.0  |
-| `repr(align)`        | 1.25.0  |
-| `extra_traits`       | 1.25.0  |
-| `core::ffi::c_void`  | 1.30.0  |
-| `repr(packed(N))`    | 1.33.0  |
-| `cfg(target_vendor)` | 1.33.0  |
-| `const-extern-fn`    | 1.62.0  |
+The minimum supported Rust toolchain version is currently **Rust 1.63**.
+
+Increases to the MSRV is allowed to change without a major (i.e. semver-
+breaking) release in order to avoid a ripple effect in the ecosystem.
+
+`libc` may continue to compile with Rust versions older than the current MSRV
+but this is not guaranteed.
+
+If your use case requires support for Rust versions below 1.63, it is
+recommended to pin `libc` with `"<=VERSION_BEFORE_THIS_MERGES"`.
 
 ## Platform support