docs: fix Base.Libc.memcmp compat note (1.9 → 1.10) (#61882)

The compat block on `Base.Libc.memcmp` has a mismatched header and body
the header reads `!!! compat "Julia 1.10"` but the body text says
"requires at least Julia 1.9".

This PR aligns the body with the header and the actual release history.

`memcmp` (and the other `mem*` methods added in #49550) first shipped in
`v1.10.0-alpha1`, not `v1.9.0`:

    $ git tag --contains f8dd16e5c1 | sort -V | head -3
    v1.10.0
    v1.10.0-alpha1
    v1.10.0-beta1

(`f8dd16e5c1` is the merge commit for #49550.)

One-line text edit; no doctest or behaviour change.