Update flutter/flutter ci.yaml to test against Java 25 - #193468

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Update flutter/flutter `ci.yaml` to test against Java 25.

Changes explained:

- `FlutterPluginUtilsTest`: the IOException test mocked `java.io.File`
and relied on `FileInputStream(File)` calling `file.getPath()` first. In
JDK 25, the constructor calls the package-private `File.isInvalid()`
first, so the strict MockK mock threw a `MockKException` instead of the
expected `IOException`. The test now uses a real directory named
`build.gradle`. Reading a directory throws `FileNotFoundException` on
every JDK and platform.
- JDK 21 `FileInputStream(File)` calls `getPath()` first
([source](https://github.com/openjdk/jdk/blob/jdk-21-ga/src/java.base/share/classes/java/io/FileInputStream.java#L136-L146)).
JDK 25 calls `isInvalid()` first
([source](https://github.com/openjdk/jdk/blob/jdk-25-ga/src/java.base/share/classes/java/io/FileInputStream.java#L132-L136)).
- The JDK 25 Javadoc says a `FileNotFoundException` is thrown if the
file "is a directory rather than a regular file"
([source](https://github.com/openjdk/jdk/blob/jdk-25-ga/src/java.base/share/classes/java/io/FileInputStream.java#L120-L126)).
- Bump MockK `1.13.16` → `1.14.11` in the Flutter Gradle Plugin. MockK
bundles Byte Buddy for inline mocking, and Byte Buddy must support the
JDK that runs the tests (Java 25 requires Byte Buddy >= 1.17.5). MockK
1.13.16 depends on Byte Buddy 1.14.17, which rejects Java 25 class files
("Java 25 (69) is not supported by the current version of Byte Buddy").
That broke 29 FGP unit tests on JDK 25. MockK 1.14.11 depends on Byte
Buddy 1.18.2.
- Byte Buddy 1.17.5 release notes: "Update ASM to version 9.8 to support
Java 25" ([release
notes](https://github.com/raphw/byte-buddy/blob/master/release-notes.md)).
- MockK 1.13.16 uses Byte Buddy 1.14.17
([POM](https://repo1.maven.org/maven2/io/mockk/mockk-agent-jvm/1.13.16/mockk-agent-jvm-1.13.16.pom)).
MockK 1.14.11 uses Byte Buddy 1.18.2
([POM](https://repo1.maven.org/maven2/io/mockk/mockk-agent-jvm/1.14.11/mockk-agent-jvm-1.14.11.pom)).
- Bump `mockito-core` `5.8.0` → `5.20.0` in `integration_test`. Mockito
bundles Byte Buddy, which must support the JDK running the tests.
Mockito 5.8.0 depends on Byte Buddy 1.14.10, which rejects Java 25 class
files, so `FlutterDeviceScreenshotTest` failed on JDK 25. Mockito 5.20.0
depends on Byte Buddy 1.17.7. This is the same version the engine's
Android `test_runner` uses.
- Mockito 5.8.0 uses Byte Buddy 1.14.10
([POM](https://repo1.maven.org/maven2/org/mockito/mockito-core/5.8.0/mockito-core-5.8.0.pom)).
Mockito 5.20.0 uses Byte Buddy 1.17.7
([POM](https://repo1.maven.org/maven2/org/mockito/mockito-core/5.20.0/mockito-core-5.20.0.pom)).
- Regenerated the 12 `project-integration_test.lockfile` files for the
Mockito bump. Each changes `mockito-core` 5.8.0 → 5.20.0 and
`byte-buddy` / `byte-buddy-agent` 1.14.10 → 1.17.7.

Partially Addresses flutter#193404

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