libksba: updated to 1.6.0

Noteworthy changes in version 1.6.0 (2021-06-10) [C22/A14/R0]
------------------------------------------------

* Limited support for the Authenticated-Enveloped-Data content type.
  [81fdcd680c12]

* Support password based decryption.  [cb7f2484a09c]

* Fix build problem on macOS.

* Silence warnings from static analyzers.

* Interface changes relative to the 1.5.0 release:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  KSBA_CT_AUTHENVELOPED_DATA      NEW.

Release-info: https://dev.gnupg.org/T5479

Noteworthy changes in version 1.5.1 (2021-04-06) [C21/A13/R1]
------------------------------------------------

* Support Brainpool curves specified by ECDomainParameters.

Release-info: https://dev.gnupg.org/T5379

Noteworthy changes in version 1.5.0 (2020-11-18) [C21/A13/R0]
------------------------------------------------

* ksba_cms_identify now identifies OpenPGP keyblock content.

* Supports TR-03111 plain format ECDSA signature verification.

* Fixes a CMS signed data parser bug exhibited by a somewhat strange
  CMS message.  [b6438e768c]

* Interface changes relative to the 1.4.0 release:
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  KSBA_CT_OPENPGP_KEYBLOCK        NEW.

Release-info: https://dev.gnupg.org/T5146

(adam)