docs: drop blanket statement recommending against remote storage

This is basically semantic revert of e5c0d80c ("docs: add note for not
using remote storages") that, while well intended, has a few problems,
e.g.:
- This is the minimal/recommended requirements section, which should
  list the rough basic specs a setup must/should have. Listing
  everything that is not best to do would bloat this list
  significantly and it's just the wrong place for it, i.e., it isn't a
  recommended against list.
- while it's true that a remote storage will basically always have
  _some_ overhead over using the same HW with a (modern) local storage
  (file) system, that does **not** mean that the remote storage has
  insufficient performance characteristics. We know of lots of fast
  Ceph setups, even release benchmarks for them, or storages like
  BlockBridge, that provide high performance while being remote.

So avoid this X-Y-problem style argumentation and focus on what is
actually important, even though I naturally get that there are some
users that use slow NFS attached storages, but breaking style here
won't cure them and I'm sure that they are capable of setting up such
a slow local storage that it won't make a real difference compared to
the NFS one.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/system-requirements.rst

@@ -38,12 +38,10 @@ Recommended Server System Requirements
 
 * Backup storage:
 
-  * Use only SSDs, for best results
+  * Prefer fast storage that delivers high IOPS for random IO workloads; use
+    only enterprise SSDs for best results.
   * If HDDs are used: Using a metadata cache is highly recommended, for example,
     add a ZFS :ref:`special device mirror <local_zfs_special_device>`.
-  * While it's technically possible to use remote storages such as NFS or SMB,
-    the additional latency and overhead drastically reduces performance and it's
-    not recommended to use such a setup.
 
 * Redundant Multi-GBit/s network interface cards (NICs)