Doubly-escaped HTML special characters in Plain Text blocks, also in old posts

### Steps to Reproduce

1. Create a Syntaxhighligher block containing the following:

```

bar &
```

2. Choose type "Plain Text".
3. Preview or publish

### What I Expected

Content displayed as:

```

bar &
```

### What Happened Instead

Content displayed as:

```
<foo>
bar &
```

### PHP / WordPress / SyntaxHighlighter Evolved version

* Debian 12
* PHP 8.2
* WordPress 6.7.2 running Twenty Fourteen theme
* SyntaxHighlighter Evolved 3.7.2

### Browser / OS version

* Any browser on any OS

### Screenshot / Video

~~A degraded public post from 2024: https://binblog.de/2024/03/04/failsafe-curl/~~ (No longer visible. Blog rolled back to 3.7.1.)

Reproducer from above:


### Context / Source

Noticed just recently. Display degraded on all existing posts. Rolled back to version 3.7.1.

No source file or test is named in the report. Reproduce the issue with a SyntaxHighlighter block set to Plain Text using the supplied and ampersand input, then compare version 3.7.2 with 3.7.1; done means previewed and published content displays each HTML special character only once escaped.

Written by the indexing model from the issue text.