PS-11473 [8.0]: azure-pipelines: build five configs by default

Cut Azure Pipelines usage on pull requests, and make the full compiler matrix
something CI can ask for explicitly.

- Drop the CI trigger (`trigger: none`); pushes no longer queue builds. The
  nightly 1:00 AM UTC schedule for 8.0 and PR validation are unaffected.
- Skip PR validation while a pull request is a draft (`pr: drafts: false`);
  builds start when it is marked ready for review.
- Replace the `clang-22 Debug INVERTED` matrix leg with `gcc-16 Debug
  INVERTED`, dropping the now-inert `UBUNTU_CODE_NAME` (it only feeds the
  apt.llvm.org repository line, which is guarded on clang).
- Build only clang-22 RelWithDebInfo, clang-22 Debug, gcc-16 RelWithDebInfo,
  gcc-16 Debug and gcc-16 Debug INVERTED by default. The other 37 legs keep a
  per-leg compile-time guard, now on a queue-time parameter:

      ${{ if or(parameters.fullCI, eq(variables['Build.Reason'], 'Schedule')) }}

This replaces the "fullci" branch-name escape hatch, which could not work. A
guard is evaluated during template expansion, and the branch is not knowable
then. Measured on a pull request from branch PS-11473-8.4-fullci, the
compile-time values are:

  Build.Reason                     'PullRequest'
  Build.SourceBranch               'refs/pull/6116/merge'
  Build.SourceBranchName           'merge'
  System.PullRequest.SourceBranch  ''            (runtime: the branch name)

So Build.SourceBranchName is always "merge" on a pull request and the old
expression never fired there; it only ever worked for non-PR runs, which its
Build.Reason term already covered.

A parameter is resolved when the run is created and therefore is visible to
${{ }}. It makes the pipeline callable from GitHub Actions in either mode:

  POST https://dev.azure.com/it0639/percona-server/_apis/pipelines/<id>/runs?api-version=7.1
  {
    "resources": { "repositories": { "self": { "refName": "refs/pull/<n>/merge" } } },
    "templateParameters": { "fullCI": true }
  }

Queued without templateParameters it builds the five default configs; with
fullCI true, all 42. The guard tests the parameter rather than "not a pull
request" because an API-created run reports Build.Reason=Manual, which would
otherwise make every queued run build everything. Schedule is kept so the
nightly still covers all 42; a manual queue from the Azure UI now gets five
unless the checkbox is ticked.

Guards stay per-leg rather than grouped under a single ${{ if }}: that is the
shape this pipeline used before and is known to expand.

Verified by expanding both branches of the guard: the default run yields the
five configs and a fullCI run yields the same 42 legs as before, with the
job's steps, variables, pool and timeout unchanged.

Backport of de116cd from 8.4. The 8.0 matrix is larger (clang-12..22,
gcc-9..16 -- 42 legs against 8.4's 36), so the extended tier is 37 legs; the
five default configs are the same.

Author: inikep