Merge remote-tracking branch 'remotes/thuth-gitlab/tags/pull-request-2021-06-02'...

* Update the references to some doc files (use *.rst instead of *.txt)
* Bump minimum versions of some requirements after removing CentOS 7 support

# gpg: Signature made Wed 02 Jun 2021 08:12:18 BST
# gpg:                using RSA key 27B88847EEE0250118F3EAB92ED9D774FE702DB5
# gpg:                issuer "thuth@redhat.com"
# gpg: Good signature from "Thomas Huth <th.huth@gmx.de>" [full]
# gpg:                 aka "Thomas Huth <thuth@redhat.com>" [full]
# gpg:                 aka "Thomas Huth <huth@tuxfamily.org>" [full]
# gpg:                 aka "Thomas Huth <th.huth@posteo.de>" [unknown]
# Primary key fingerprint: 27B8 8847 EEE0 2501 18F3  EAB9 2ED9 D774 FE70 2DB5

* remotes/thuth-gitlab/tags/pull-request-2021-06-02:
  configure: bump min required CLang to 6.0 / XCode 10.0
  configure: bump min required GCC to 7.5.0
  configure: bump min required glib version to 2.56
  tests/docker: drop CentOS 7 container
  tests/vm: convert centos VM recipe to CentOS 8
  crypto: drop used conditional check
  crypto: bump min gnutls to 3.5.18, dropping RHEL-7 support
  crypto: bump min gcrypt to 1.8.0, dropping RHEL-7 support
  crypto: drop back compatibility typedefs for nettle
  crypto: bump min nettle to 3.4, dropping RHEL-7 support
  patchew: move quick build job from CentOS 7 to CentOS 8 container
  block/ssh: Bump minimum libssh version to 0.8.7
  docs: fix references to docs/devel/s390-dasd-ipl.rst
  docs: fix references to docs/specs/tpm.rst
  docs: fix references to docs/devel/build-system.rst
  docs: fix references to docs/devel/atomics.rst
  docs: fix references to docs/devel/tracing.rst

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --cc .gitlab-ci.d/containers.yml
Simple merge

diff --cc target/i386/trace-events
Simple merge