CI: Add Erlang/OTP 28, but disable make spec (processone/ejabberd#4352)

"make spec" in xmpp fails with OTP 28, with an error similar to
"make options" in ejabberd:

   {'EXIT',
    {undef,
     [{erl_types,
       t_opaque_from_records,
       [#{{opaque,xmlel,[]} =>
           {{fxml,1,2,[]},type}}],
       []},

Author: badlop

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        otp: [20, 25, 26, 27]
+        otp: [20, 25, 26, 27, 28]
     runs-on: ubuntu-24.04
     container:
       image: erlang:${{ matrix.otp }}
@@ -21,6 +21,7 @@ jobs:
       if: matrix.otp >= 22
     - run: REBAR=rebar3 ERLTIDY=true make
     - run: REBAR=rebar3 make spec
+      if: matrix.otp < 28
     - run: REBAR=rebar3 make xdata
     - run: REBAR=rebar3 make
     - run: REBAR=rebar3 make xref