chore(provider): mark deprecation for service_account_token

[h3adex]

Description

See
https://docs.stackit.cloud/stackit/en/release-notes-23101442.html
https://docs.stackit.cloud/stackit/en/deprecation-plan-for-service-account-access-tokens-and-migration-guide-373293307.html

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

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[rubenhoenle]

There are some further adjustments needed:

terraform-provider-stackit/CONTRIBUTION.md

Line 124 in 54b483d

1. Setup authentication by setting the env var `STACKIT_SERVICE_ACCOUNT_TOKEN` as a valid token (see [Authentication](#authentication) for more details on how to autenticate).

(contributors should use keyflow instead)

---

terraform-provider-stackit/examples/provider/provider.tf

Line 10 in 54b483d

service_account_token = var.service_account_token

(token flow must be marked as deprecated here or can be removed at all)

---

terraform-provider-stackit/templates/index.md.tmpl

Line 14 in 54b483d

- Token flow

This is the initial page of the Terraform provider docs, see https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs

terraform-provider-stackit/templates/index.md.tmpl

Lines 87 to 93 in 54b483d

	### Token flow
	Using this flow is less secure since the token is long-lived. You can provide the token in several ways:
	1. Setting the field `service_account_token` in the provider
	2. Setting the environment variable `STACKIT_SERVICE_ACCOUNT_TOKEN`
	3. Setting it in the credentials file (see above)

[rubenhoenle]

see above

[rubenhoenle]

@h3adex fixed my findings for you :)

fixed