Drop core18 extensions

[mr-cal]

What needs to get done

Remove legacy core18 extensions from snapcraft_legacy/internal/project_loader/_extensions.

Why it needs to get done

snapcraft extensions shows extensions for core18, which are not usable in snapcraft 8.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3881.

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[Abdulwahab03]

Hi @mr-cal, I would like take up this issue can you please assign it to me

[mr-cal]

@Abdulwahab03, I've assigned it to you. Thanks!

Let me know if you have any questions.

I expect this will require:

• removing extensions only for core18 and anywhere they are imported or referenced
• removing unit tests of extensions only for core18
• removing core18 from get_supported_bases() for extensions that support both core18 and core20
• updating unit tests as needed