Provide more specific error message for StrictUsedVariable check based on unused variable type

[IlanaRadinsky]

Fixes #2361

Before this PR

Turns out this was just a confusing error message. Previously, a code snippet like

public class Test {
    private final String myField;

    public Test(String myField) {
        this.myField = myField;
    }
}

would result in an error message

error: [StrictUnusedVariable] The field 'field' is never read. Intentional occurrences are acknowledged by renaming the unused variable with a leading underscore. '_field', for example.
  private String field;
                 ^
    (see https://github.com/palantir/gradle-baseline#baseline-error-prone-checks)
  Did you mean to remove this line?

If you applied the fix from the error message, checks would fail because the field is read but has 'StrictUnusedVariable' suppressed because of its name (prefixed with an underscore).

But really, the correct fix is suggested below the code snippet/link - to remove the field (Did you mean to remove this line?).

After this PR

==COMMIT_MSG==
Provide more specific error message for StrictUsedVariable check based on unused variable type
==COMMIT_MSG==

Possible downsides?

[changelog-app]

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• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

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• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Provide more specific error message for StrictUsedVariable check based on unused variable type

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[IlanaRadinsky]

This only reports a match if the var is a local var, so no need to do it for all vars (including parameters) as we were before.