Remove Service accounts from premium features

What does this MR do and why?

Remove :service_accounts from PREMIUM_FEATURES and license gates

Remove :service_accounts from PREMIUM_FEATURES in features.rb, making service accounts a core feature available to all tiers including CE and unlicensed self-managed instances.

Refactor Authn::ServiceAccounts to eliminate all license-based gating:

• available? and available_for_namespace? now return true unconditionally
• free_tier? and free_tier_namespace? now derive free tier status from license/subscription plan rather than feature availability
• creation_allowed_for_sm? and creation_allowed_for_saas? retain seat count limits for free tier (100) and trials, with the allow_unlimited_service_account_for_trials FF preserved
• Remove service_accounts_available_on_free_or_unlicensed FF references from Authn::ServiceAccounts
• FF service_accounts_available_on_free_or_unlicensed definition cleanup is also included

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/592763

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Related to #592763