ci: remove ubuntu-xenial from tests

Ubuntu Xenial now consistently crashes in our virtualization setup with the
following message:

  [    1.105749] kernel BUG at /build/linux-3Zqs18/linux-4.4.0/arch/x86/kernel/alternative.c:709!
  [    1.107533] invalid opcode: 0000 [#1] SMP
  [    1.108637] Modules linked in:
  [    1.109471] CPU: 0 PID: 106 Comm: systemd-udevd Not tainted 4.4.0-210-generic #242-Ubuntu
  [    1.111225] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
  [    1.113159] task: ffff880036f7e3c0 ti: ffff880037038000 task.ti: ffff880037038000
  [    1.114808] RIP: 0010:[<ffffffff81036b43>]  [<ffffffff81036b43>] text_poke+0x1a3/0x240
  [    1.116609] RSP: 0018:ffff88003703bc60  EFLAGS: 00010007
  [    1.117717] RAX: 00000000000000f0 RBX: 0000000000000800 RCX: 00000000fffa3203
  [    1.119118] RDX: 00000000078bfbff RSI: 0000000000000000 RDI: ffffffffff572000
  [    1.120527] RBP: ffff88003703bc98 R08: ffffffff81075060 R09: 0000000000000000
  [    1.121928] R10: ffffffff81f43640 R11: ffffffff81f43600 R12: ffffffffc00031a3
  [    1.123317] R13: 0000000000000001 R14: ffff88003703bcbf R15: 0000160000000000
  [    1.124730] FS:  00007f3de7b188c0(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000
  [    1.126467] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [    1.127645] CR2: 00007f3de7b17e31 CR3: 0000000037028000 CR4: 00000000003606f0
  [    1.129036] Stack:
  [    1.129624]  0000000000000286 ffffea0000dbb680 ffffffffc0004174 ffffffffc0002000
  [    1.131595]  ffffffffc0003b29 ffffffffc0005458 ffffffffc0004164 ffff88003703bcf0
  [    1.133572]  ffffffff81036d2b 0000000081dba540 ffffffffc0005440 3effffffc0005440
  [    1.135548] Call Trace:
  [    1.136206]  [<ffffffff81036d2b>] alternatives_smp_module_add+0x14b/0x190
  [    1.137554]  [<ffffffff81063f26>] module_finalize+0x136/0x150
  [    1.138734]  [<ffffffff81112216>] load_module+0x13e6/0x1c80
  [    1.139886]  [<ffffffff81222b50>] ? kernel_read+0x50/0x80
  [    1.141004]  [<ffffffff8110ed6d>] ? copy_module_from_fd.isra.55+0xfd/0x150
  [    1.142518]  [<ffffffff81112cf4>] SYSC_finit_module+0xb4/0xe0
  [    1.143714]  [<ffffffff81112d3e>] SyS_finit_module+0xe/0x10
  [    1.144872]  [<ffffffff8186bb5b>] entry_SYSCALL_64_fastpath+0x22/0xd0
  [    1.146162] Code: 00 00 0f a2 4d 85 ed 74 4a 41 0f b6 04 24 41 38 06 75 18 31 d2 83 c2 01 48 63 c2 49 39 c5 76 33 41 0f b6 0c 06 41 38 0c 04 74 ea <0f> 0b 4c 89 e7 e8 63 38 1a 00 49 8d bc 24 00 10 00 00 48 89 c3
  [    1.155531] RIP  [<ffffffff81036b43>] text_poke+0x1a3/0x240
  [    1.156761]  RSP <ffff88003703bc60>

Since we don't really need to support Ubuntu 16.04, just remove it from the
list of distributions to test.

.gitlab-ci.yml

@@ -109,7 +109,8 @@ e2e-test:
     # debian-9: no longer supported, and cloud-init so old it crashes when a network-config is provided
     # rocky-8: requires "--secure-boot=true" but we do not yet have a way of specifying that
     # rocky-9: requires "--secure-boot=true" but we do not yet have a way of specifying that
-    EXCLUDED: amazonlinux-2|centos-6|centos-8|debian-9|rocky-8|rocky-9
+    # ubuntu-xenial: kernel crashes on boot
+    EXCLUDED: amazonlinux-2|centos-6|centos-8|debian-9|rocky-8|rocky-9|ubuntu-xenial
   rules:
     - if: $CI_MERGE_REQUEST_ID
   tags: