Catch wrong use of `==`/`!=` and `equals()`/`!equals()` in equals() implementations

Bugs like this: #8326 could be caught on the static analysis level, using Regex patterns ([Checkstyle Regexp](https://checkstyle.org/config_regexp.html)) or [Structural search inspection](https://github.com/apache/incubator-druid/blob/master/dev/teamcity.md#creating-a-custom-inspection-from-a-structural-search-pattern).

Example of regex which should have caught #8326: `(?m)return ([^\;]+)(this\.)?([a-zA-Z]+) \!\= [a-zA-Z]+\.\1`

Similar regexes could be created for `equals()`, `Object.equals()`, as well as incorrect use in
```java
if (x != that.x) {
return false;
}
```
code in `equals()` implementations.

Start by reviewing the linked Checkstyle Regexp documentation, the structural-search inspection guidance in dev/teamcity.md, and the buggy example in issue #8326. Define the checks for ==/!= and equals()/!equals() inside equals() implementations, then verify that the resulting patterns catch the example without unacceptable false positives.

Written by the indexing model from the issue text.