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>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 203d628..3498ac5 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -59,7 +59,7 @@
 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.