[7.x] Reintroduce parser error for mixed pipe/shorthand modifier syntax

[marcorieser]

#14892 threw a parser error when a tag mixed pipe (|) and shorthand key=value modifier syntax, fixing #13080. Reverted in #15073 — it broke previously working syntax like {{ last_modified | format="U" }}, a breaking change for a minor release.

Mixing both styles is always invalid and should always throw, e.g.:

{{ active_platforms | sort="value" }}
    ...
{{ /active_platforms }}

{{ last_modified | format="U" }}

The second case only "worked" because the pipe segment was silently ignored while the shorthand param did the real work — worth disallowing.

Tasks:

• Bring back the logic from [6.x] Throw parser error when mixing pipe and shorthand modifier syntax #14892 -> [7.x] Reintroduce mixed pipe shorthand modifier error #15075
• Drop the regression test
• Mention the changes in the upgrade guide

Related: #13080, #14892, #15073