Fix error BuildpackApi message

It looks like the message for BuildpackApi was copied from BuildpackId.

From the spec https://github.com/buildpacks/spec/blob/main/buildpack.md#buildpack-api-version

```
This document specifies Buildpack API version 0.5

Buildpack API versions:

    MUST be in form <major>.<minor> or <major>, where <major> is equivalent to <major>.0
    When <major> is greater than 0 increments to <minor> SHALL exclusively indicate additive changes
```

The spec actually doesn't say that it should only be numbers, but really it should. To be extremely specific, we can also specify that it cannot be greater u32::MAX which would make our implementation standards compliant.

src/data/buildpack.rs

@@ -253,7 +253,7 @@ impl StackId {
 
 #[derive(thiserror::Error, Debug)]
 pub enum BuildpackTomlError {
-    #[error("Found `{0}` but value MUST only contain numbers, letters, and the characters `.`, `/`, and `-`. Value MUST NOT be 'config' or 'app'.")]
+    #[error("Found `{0}` but value MUST be in the form `<major>.<minor>` or `<major>` and only contain numbers.")]
     InvalidBuildpackApi(String),
 
     #[error("Stack with id `*` MUST not contain mixins. mixins: [{0}]")]