ci: drop debian 9

It looks like downloads won't work anymore:
 https://gitlab.nic.cz/knot/knot-resolver/-/jobs/890201
 https://gitlab.nic.cz/knot/knot-resolver/-/jobs/890312
which is probably because long-term support ended last summer.

.gitlab-ci.yml

@@ -653,13 +653,6 @@ obs:rocky8:x86_64:
     DISTROTEST_NAME: rocky8
     DISTROTEST_REPO: CentOS_8_EPEL
 
-obs:debian9:x86_64:
-  <<: *distrotest
-  variables:
-    OBS_REPO: knot-resolver-latest
-    DISTROTEST_NAME: debian9
-    DISTROTEST_REPO: Debian_9.0
-
 obs:debian10:x86_64:
   <<: *distrotest
   only:

ci/pkgtest.yaml

@@ -51,15 +51,6 @@ centos-7:pkgbuild:
     - yum install -y rpm-build python3-pip epel-release
     - *apkgbuild
 
-debian-9:pkgbuild:
-  <<: *pkgbuild
-  image: $CI_REGISTRY/labs/lxc-gitlab-runner/debian-9
-  variables:
-    OBS_REPO: knot-resolver-build
-    DISTROTEST_REPO: Debian_9.0
-  script:
-    - *debpkgbuild
-
 debian-10:pkgbuild:
   <<: *pkgbuild
   image: $CI_REGISTRY/labs/lxc-gitlab-runner/debian-10
@@ -169,17 +160,6 @@ centos-7:pkgtest:
     - systemctl start kresd@1
     - kdig @127.0.0.1 nic.cz | grep -qi NOERROR
 
-debian-9:pkgtest:
-  <<: *pkgtest
-  needs:
-    - debian-9:pkgbuild
-  image: $CI_REGISTRY/labs/lxc-gitlab-runner/debian-9
-  variables:
-    OBS_REPO: knot-resolver-build
-    DISTROTEST_REPO: Debian_9.0
-  script:
-    - *debpkgtest
-
 debian-10:pkgtest:
   <<: *pkgtest
   needs: