docs: document S3 SSE-KMS support and deprecate SSE-C

[cuppett]

Add documentation for the SSE-KMS (AWS Key Management Service) server-side encryption
support introduced in Nextcloud 34 (nextcloud/server#57623). Mark SSE-C as deprecated,
noting the AWS policy change that disabled SSE-C by default on new S3 buckets in April
2026 and recommending SSE-KMS for new deployments.

Changes:

• primary_storage.rst — adds sse_kms_enabled and sse_kms_key_id to the S3
  optional parameters list; marks the SSE-C section .. deprecated:: 34 with a
  .. warning:: about the AWS policy change; adds a new "S3 SSE-KMS encryption support"
  section (.. versionadded:: 34) with two configuration examples (specific KMS key ARN
  and bucket-default key); also fixes the SSE-C code example (replaces array() syntax
  with [] and switches the bare :: blocks to .. code-block:: php/bash)
• config_sample_php_parameters.rst — adds sse_kms_enabled, sse_kms_key_id, and
  sse_c_key with descriptive inline comments to the S3 objectstore sample block
• encryption_configuration.rst — expands the SSE note to cross-reference both SSE-KMS
  (recommended) and the deprecated SSE-C sections via :ref: and :doc: links

☑️ Resolves

• Documents feat(objectstore): Add AWS SSE-KMS encryption support for S3 storage server#57623

🖼️ Screenshots

✅ Checklist

• I have built the documentation locally and reviewed the output
• Screenshots are included for visual changes
• I have not moved or renamed pages (or added a redirect if I did)
• I have run codespell or similar and addressed any spelling issues