Replace removed crypto:random_bytes/1 with crypto:strong_rand_bytes/1

[tas50]

Problem

bkswt_api_SUITE:test_data/1 calls crypto:random_bytes/1, which was removed from OTP years ago and does not exist in any currently supported release. Calling it raises undef — on the pinned OTP 26 today, and equally on 27 and 28:

1> crypto:random_bytes(4).
** exception error: undefined function crypto:random_bytes/4
   {'EXIT', {undef, [{crypto,random_bytes,[4],[]}, ...]}}

Two things have kept this hidden:

1. Remote calls are not resolved at compile time, so the build never flags it.
2. test_data/1 is currently unreferenced within the suite (only its sibling test_data_text/1 is called), and it survives the "function unused" check only because the suite sets -compile(export_all).

It is therefore a latent landmine rather than a live failure — but it is exactly the kind of thing worth clearing out before an OTP upgrade, since anyone wiring test_data/1 into a case would hit an undef that looks like an OTP regression.

Fix

crypto:strong_rand_bytes/1 is the supported replacement, and is already what bookshelf_bench uses to build test payloads of a given size (bookshelf_bench.erl:109 and :149), so this keeps the helper consistent with the rest of the app.

 test_data(Size) ->
-    crypto:random_bytes(Size).
+    crypto:strong_rand_bytes(Size).

How this was found

While auditing the Erlang sources ahead of an OTP upgrade, I checked every module:function pair in the tree against the exports of a real OTP 28 runtime. Nine pairs came back unresolved; eight were remote type references in specs and records (calendar:datetime(), dict:dict(), file:io_device(), file:posix(), re:mp(), proplists:proplist(), gb_trees:tree(), calendar:datetime1970()), which are all legitimate. crypto:random_bytes was the only genuine hit in the entire tree.