Move storefront translations out of `solidus_core`

Desired Behavior

All translations used by the starter frontend reside in solidus_core: This creates challenges during the development of the starter, as each time a translation key needs to be added, a dedicated PR in solidus is required.

Moreover, it's not consistent to have these translations in the core: They should be part of the frontend, which will then be copied into the host after installation, allowing the possibility to make changes if needed.

The plan is to conduct this transition in different stages across both repositories and:

1. Move all the deprecated translations in core to a separate locale files (e.g. solidus/core/config/locales/en.deprecated.yml).
2. Copy the file to the host application as part of the solidus installation/upgrade process.
3. In the next major release, remove the deprecated file from solidus_core and relocate it to solidus_starter_frontend.

For more details, see this conversation: https://github.com/solidusio/solidus_starter_frontend/pull/348#discussion_r1250583682

Current Behavior

Currently, any addition or change to translations requires a new pull request in solidus, as all translation keys used by the starter frontend are located in solidus_core.

Additional notes

Consider this specific key, which hasn't been added to solidus_core to avoid the needless inclusion of another key that would subsequently need to be removed: https://github.com/solidusio/solidus/pull/5208
This key will be among those directly inserted into the solidus_starter_frontend.

Review the discussion in solidus_starter_frontend PR #348 and inspect the translation files under solidus/core/config/locales, including the proposed en.deprecated.yml. Trace the Solidus installation and upgrade process to determine where the file would be copied, then verify that the staged move covers deprecated keys and the later relocation to solidus_starter_frontend.