feature `procmacro2_semver_exempt` does not exist: `Span::eq()` is documented but not available

[lib.rs: 548](https://github.com/dtolnay/proc-macro2/blob/bb38d83b86d2bfb8a31ff81cd540b36de1b236ee/src/lib.rs#L548) declares

```rust
/// Compares two spans to see if they're equal.
///
/// This method is semver exempt and not exposed by default.
#[cfg(procmacro2_semver_exempt)]
#[cfg_attr(docsrs, doc(cfg(procmacro2_semver_exempt)))]
pub fn eq(&self, other: &Span) -> bool {
self.inner.eq(&other.inner)
}
```
(which is also then documented on docs.rs)

but no feature `procmacro2_semver_exempt` is made available via `Cargo.toml` leading to:

```
error: failed to select a version for `proc-macro2`.
... required by package `proc_macro2_diagnostic v0.3.1 (/workspaces/proc_macro2_diagnostic)`
... which satisfies path dependency `proc_macro2_diagnostic` of package `proc_macro2_diagnostic_fixture v0.0.1 (/workspaces/proc_macro2_diagnostic/tests/compilation)`
versions that meet the requirements `^1.0.106` are: 1.0.106

package `proc_macro2_diagnostic` depends on `proc-macro2` with feature `procmacro2_semver_exempt` but `proc-macro2` does not have that feature.
help: available features: default, nightly, proc-macro, span-locations
```

I would be happy to offer a (non-AI) PR to either remove `Span::eq` or add feature `procmacro2_semver_exempt`