fix: re-enable sqlserverflex flavor (#108)

## Description

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relates to #1234

## Checklist

- [ ] Issue was linked above
- [ ] Code format was applied: `make fmt`
- [ ] Examples were added / adjusted (see `examples/` directory)
- [x] 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](f5f99d1709/stackit/internal/services/dns/dns_acc_test.go))
- [x] Unit tests are passing: `make test` (will be checked by CI)
- [x] No linter issues: `make lint` (will be checked by CI)

Co-authored-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>
Reviewed-on: #108

docs/resources/sqlserverflexbeta_instance.md

@@ -97,7 +97,6 @@ import {
 ### Required
 
 - `backup_schedule` (String) The schedule for on what time and how often the database backup will be created. The schedule is written as a cron schedule.
-- `flavor_id` (String) The id of the instance flavor.
 - `name` (String) The name of the instance.
 - `network` (Attributes) the network configuration of the instance. (see [below for nested schema](#nestedatt--network))
 - `retention_days` (Number) The days for how long the backup files should be stored before cleaned up. 30 to 365
@@ -107,6 +106,8 @@ import {
 ### Optional
 
 - `encryption` (Attributes) this defines which key to use for storage encryption (see [below for nested schema](#nestedatt--encryption))
+- `flavor` (Attributes, Deprecated) (see [below for nested schema](#nestedatt--flavor))
+- `flavor_id` (String) The id of the instance flavor.
 - `instance_id` (String) The ID of the instance.
 - `project_id` (String) The STACKIT project ID.
 - `region` (String) The region which should be addressed
@@ -156,3 +157,17 @@ Required:
 - `kek_key_ring_id` (String) The keyring identifier
 - `kek_key_version` (String) The key version
 - `service_account` (String)
+
+
+<a id="nestedatt--flavor"></a>
+### Nested Schema for `flavor`
+
+Optional:
+
+- `cpu` (Number, Deprecated)
+- `ram` (Number, Deprecated)
+
+Read-Only:
+
+- `description` (String)
+- `id` (String)