package.json license uses incorrect SPDX code (MPL-2.0 instead of MPL-2.0-no-copyleft-exception)

### Product

axe-core & other related Deque open source libraries

### Product Version

4.10.2

### Latest Version

- [x] I have tested the issue with the latest version of the product

### Issue Description

Followup to https://github.com/dequelabs/axe-core/issues/4680#issuecomment-2631967902

axe-core's [LICENSE](https://github.com/dequelabs/axe-core/blob/develop/LICENSE) is MPL-2.0 *with Exhibit B* (and has been for 10 years).

However, [`package.json`'s `license` field currently lists `"MPL-2.0"`](https://github.com/dequelabs/axe-core/blob/9a743ee298df3ac300006335128bbdd1ca63ccd5/package.json#L5). Per spdx.org, [the `MPL-2.0` SPDX code](https://spdx.org/licenses/MPL-2.0.html) is "for use when the standard MPL 2.0 is used, as indicated by the standard header (Exhibit A but **no Exhibit B**)" (emphasis mine), and the correct SPDX code for MPL 2.0 *with* exhibit B is [`MPL-2.0-no-copyleft-exception`](https://spdx.org/licenses/MPL-2.0-no-copyleft-exception.html).

This issue tracks that we need to update the SPDX code to match the actual license. This would *not* be a relicensing of a project (it is *already* licensed *with* exhibit B, per its `LICENSE` file), but we'll want to discuss and consider what kind of version number we want to put this update behind, since it is likely to be perceived as breaking by some users even if we believe it is more of a bugfix than a breaking change.

We'll want to apply similar changes to our other open source libraries; most of them have the same issue.