Use spdx identifier for license name

[StefMa]

Hello mocki-devs 👋

in our company we just added licensee to our code base to make sure we are only using dependencies with licenses we agree on.

Behind the scense, the plugin scan the POM file and check the license name.
The plugin allow only license names that are part of the SPDX Specification.
For whatever reasons, we can't add any arbitrary license names to it, without adding the license URL.
This means, adding the license name The MIT License (what you are currently use) without adding the license URL (https://github.com/mockito/mockito/blob/main/LICENSE) is not working. It will fail with an error message.

Because mockito uses the default MIT License without any modification I guess it make sense to align this and use the SPDX Identifier, which is just MIT.

If we use this, the plugin will not fail anymore. Because it detects the official SPDX Identifier and is there "sure" that this is a valid license.

Honestly, I'm not sure you want this change. 😁
So feel free to close it and just go with the current license name you have been using for years.
On the other hand, why not stick to an official specification? Shouldn't hurt anyway, right? 🤷

Checklist

• Read the contributing guide
• PR should be motivated, i.e. what does it fix, why, and if relevant how
• If possible / relevant include an example in the description, that could help all readers
  including project members to get a better picture of the change
• Avoid other runtime dependencies
• Meaningful commit history ; intention is important please rebase your commit history so that each
  commit is meaningful and help the people that will explore a change in 2 years
• The pull request follows coding style
• Mention Fixes #<issue number> in the description if relevant
• At least one commit should mention Fixes #<issue number> if relevant

[TimvdLippe]

Makes sense, thanks for the fix!