Remove support for cryptography < 3.4 (#878)

* Stop passing backend to cryptography.

* Make public_bytes() fallback the default.

* Remove compatibility code for older cryptography versions.

* Require cryptography 3.4+.

* Restrict to cryptography >= 3.4 in integration tests.

* Remove Debian Bullseye from CI.

It only supports cryptography 3.3.

* Improve imports.

* Remove no longer existing conditional.

plugins/modules/x509_certificate_convert.py

@@ -85,8 +85,6 @@ seealso:
   - module: community.crypto.x509_certificate
   - module: community.crypto.x509_certificate_pipe
   - module: community.crypto.x509_certificate_info
-requirements:
-  - cryptography >= 1.6 if O(verify_cert_parsable=true)
 """
 
 EXAMPLES = r"""
@@ -132,12 +130,11 @@ from ansible_collections.community.crypto.plugins.module_utils.io import (
 )
 
 
-MINIMAL_CRYPTOGRAPHY_VERSION = "1.6"
+MINIMAL_CRYPTOGRAPHY_VERSION = "3.4"
 
 CRYPTOGRAPHY_IMP_ERR = None
 try:
     import cryptography  # noqa: F401, pylint: disable=unused-import
-    from cryptography.hazmat.backends import default_backend
     from cryptography.x509 import load_der_x509_certificate
 except ImportError:
     CRYPTOGRAPHY_IMP_ERR = traceback.format_exc()
@@ -234,7 +231,7 @@ class X509CertificateConvertModule(OpenSSLObject):
                 exception=CRYPTOGRAPHY_IMP_ERR,
             )
         try:
-            load_der_x509_certificate(self.input, default_backend())
+            load_der_x509_certificate(self.input)
         except Exception as exc:
             module.fail_json(msg=f"Error while parsing certificate: {exc}")