docs: decrease the minimum supported OpenSSL

Our code does not preclude the use of versions 1.0.x of OpenSSL.
Instead, we discourage it's use due to security concerns. Update the
documentation to reflect this.

Change-Id: I5c60907337f10b05d5c43b0384247c5d4135db50
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
(cherry picked from commit 1b86ec5b5de3866d03a6de69056dec1d8d4b3398)
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index e3b33e4..8a6c58c 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -58,7 +58,7 @@
 The following libraries must be available to build one or more components or
 supporting tools:
 
-- OpenSSL >= 1.1.1 (v3.0.0 to v3.0.6 highly discouraged due to security issues)
+- OpenSSL >= 1.0.0 (1.0.x, v3.0.0 to v3.0.6 highly discouraged due to security issues)
 
    Required to build the cert_create, encrypt_fw, and fiptool tools.