"UTC" time zone inconsistently parsed with "DateTimeFormatter" after Java11

ADDITIONAL SYSTEM INFORMATION :
Windows Server 2019, macOS 10.15, Ubuntu 18.04.
Java 11, Java 15.

A DESCRIPTION OF THE PROBLEM :
The `UTC` time zone specified in a String with date and time is parsed to a different `ZoneId` in Java11+ rather than in Java8.

Consider the following pattern `MM/dd/yyyy hh:mm a z` used to create a `DateTimeFormatter` with the US locale.

When a date string such as e.g. `05/17/2021 09:23 PM UTC` is parsed to a `ZonedDateTime` using the formatter, the `ZoneId` value of the `ZonedDateTime` is different when different JDK versions are used.

For JDK 8-10 and 12-14 the value of the `ZoneId` is `UTC`. But for JDK 11 and JDK 15 the value is equal to `Etc/UTC`.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
The easiest way to reproduce the issue is to clone the repository https://github.com/yuri-sergiichuk/jdk-zone-parsing-inconsistency and run Gradle build `./gradlew build`.

1. Create `DateTimeFormatter` with the `MM/dd/yyyy hh:mm a z` pattern:

    ```
    DateTimeFormatter ZONED_FORMATTER = DateTimeFormatter.ofPattern("MM/dd/yyyy hh:mm a z", Locale.US);
    ```

2. Parse date-time string `05/17/2021 09:23 PM UTC` to a `ZonedDateTime` object:

    ```
    ZonedDateTime result = ZONED_FORMATTER.parse(dateString, ZonedDateTime::from);
    ```

3. Check `ZoneId` value of the result:

    ```
    result.getZone().equals(ZoneId.of("UTC"))
    ```


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The parsed `ZonedDateTime` zone is equal to `ZoneId.of("UTC")`.
ACTUAL -
The parsed `ZonedDateTime` zone is equal to `ZoneId.of("Etc/UTC")`.

---------- BEGIN SOURCE ----------
final class ZonedParser {

    public static void main(String[] args) {
        DateTimeFormatter ZONED_FORMATTER = DateTimeFormatter.ofPattern("MM/dd/yyyy hh:mm a z", Locale.US);
        ZonedDateTime result = ZONED_FORMATTER.parse("05/17/2021 09:23 PM UTC", ZonedDateTime::from);
        if (!result.getZone().equals(ZoneId.of("UTC"))) {
            throw new IllegalStateException("Zone ID must be equal to `UTC`.");
        }
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use `Etc/UTC` time zone in date string in Java 11 and Java 15 and just `UTC` in others.

FREQUENCY : always