chore: removed the migration path from JiraEconomics

[tuj]

Link to ticket

https://leantime.itkdev.dk/#/tickets/showTicket/8000

Description

Removes the JiraEconomics migration path. app:migrate-from-jira-economics was a one-shot
cutover helper: it hand-created doctrine_migration_versions, marked Version20230101000000
as already run, then dropped messenger_messages and migration_versions. That cutover is
long done, and docs/migration-from-jira-economics.md was already dead — five of the six
commands it told you to run no longer exist (app:sync-projects, app:sync-accounts,
app:migrate-customers, app:sync-issues, app:sync-worklogs).

Nothing was wired to the command beyond the #[AsCommand] autowiring — no service definition,
no PHPStan baseline entry, no tests, no env vars, no separate Doctrine connection — so deleting
it is self-contained.

Also cleans up the references that would otherwise go stale:

• CLAUDE.md — dropped the sentence pointing at the deleted command and removed it from the
  console-command list (and added the missing app:anonymize-worklogs while in there).
• README.md — "e.g. Jira" → "e.g. Leantime", since Leantime is the only entry in
  DataProviderService::IMPLEMENTATIONS.
• InvoiceController and BillingService — reworded three "backwards compatible with
  JiraEconomics" comments. Comments only. The logic they guard still matters for imported
  rows: the free-text paidByAccount / defaultReceiverAccount fallback, and the INTERN /
  EKSTERN locked-type literals that recorded invoices carry.

The jira_key / jira_id columns in migrations/Version20230101000000.php and
Version20230101000002.php are left alone — executed history, and the base of the migration chain.

Checklist

• My code is covered by test cases.
• My code passes our test (all our tests).
• My code passes our static analysis suite.
• My code passes our continuous integration process.