feat!: remove deprecated CLI options

[finswimmer]

This PR removes CLI options that were introduced during development of Poetry 1.2 but get replaced until today:

• poetry plugin --> use poetry self add instead
• --default option for group commands --> use --with main instead

[mkniewallner]

I guess we should also remove mentions of --default from the documentation in those places:

• install:

  poetry/docs/cli.md

  Line 230 in a3aafa8

  * `--default`: Only include the main dependencies. (**Deprecated**)

• update:

  poetry/docs/cli.md

  Line 271 in a3aafa8

  * `--default`: Only include the main dependencies. (**Deprecated**)

• show:

  poetry/docs/cli.md

  Line 474 in a3aafa8

  * `--default`: Only include the main dependencies. (**Deprecated**)

[finswimmer]

Excellent catch @mkniewallner 👍

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