minor #11619 added useful reminder about form.vars.errors into UPGRAD…

…E-2.5 notes (Oleg Andreyev)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #11619).

Discussion
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added useful reminder about form.vars.errors into UPGRADE-2.5 notes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

PR is just about upgrading UPGRADE-2.5 notes.
We've been upgrading from 2.4 to 2.5.3 and missing this part (upgrading templates)

Commits
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aefb1ca added useful reminder about form.vars.errors into UPGRADE-2.5 notes

UPGRADE-2.5.md

@@ -45,6 +45,18 @@ Form
    {
    ```
 
+   Before:
+
+   ```
+   {% if form.vars.errors %}
+   ```
+
+   After:
+
+   ```
+   {% if form.vars.errors|length %}
+   ```
+
 PropertyAccess
 --------------