Update to Gradle 8.6 (#245)

- Fix and clean up the pkl-commons-test build script.
- Change tests to read test packages/certs directly from
  the file system instead of packaging and reading them
  from the class path.
- Update expected checksums of some test packages.
- Fix a conflict between Pkl's and Gradle's
  Kotlin libraries in the pkl-gradle project.
- Fix build deprecation warnings.
- Ensure Gradle distribution integrity with `distributionSha256Sum`.
- Manually verify integrity of Gradle wrapper added by this commit.

pkl-core/src/test/files/LanguageSnippetTests/input/projects/project4/PklProject.deps.json

@@ -5,7 +5,7 @@
       "type": "remote",
       "uri": "projectpackage://localhost:12110/fruit@1.0.5",
       "checksums": {
-        "sha256": "b4ea243de781feeab7921227591e6584db5d0673340f30fab2ffe8ad5c9f75f5"
+        "sha256": "abd173e8a25f5b930b0e34269a441e32c9d95e0b0a715bc6eff918f0afd0688e"
       }
     }
   }

pkl-core/src/test/kotlin/org/pkl/core/packages/PackageResolversTest.kt

@@ -181,7 +181,7 @@ class PackageResolversTest {
           null)
       }
       assertThat(error).hasMessageContaining("""
-        Computed checksum: "0ec8a501e974802d0b71b8d58141e1e6eaa10bc2033e18200be3a978823d98aa"
+        Computed checksum: "a6bf858cdd1c09da475c2abe50525902580910ee5cc1ff624999170591bf8f69"
         Expected checksum: "intentionally bogus checksum"
       """.trimIndent())
     }

pkl-core/src/test/kotlin/org/pkl/core/project/ProjectDependenciesResolverTest.kt

@@ -50,14 +50,14 @@ class ProjectDependenciesResolverTest {
             "type": "remote",
             "uri": "projectpackage://localhost:12110/birds@0.5.0",
             "checksums": {
-              "sha256": "3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118"
+              "sha256": "0a5ad2dc13f06f73f96ba94e8d01d48252bc934e2de71a837620ca0fef8a7453"
             }
           },
           "package://localhost:12110/fruit@1": {
             "type": "remote",
             "uri": "projectpackage://localhost:12110/fruit@1.1.0",
             "checksums": {
-              "sha256": "98ad9fc407a79dc3fd5595e7a29c3803ade0a6957c18ec94b8a1624360b24f01"
+              "sha256": "a82e92e0c259591111d09d18a14f5ad66e2b6e13d827ee3e6f7ce06f5d0fbe0c"
             }
           },
           "package://localhost:12110/project3@1": {
@@ -81,7 +81,7 @@ class ProjectDependenciesResolverTest {
     assertThat(e).hasMessage("""
       Computed checksum did not match declared checksum for dependency `package://localhost:12110/birds@0.5.0`.
 
-      Computed: "3f19ab9fcee2f44f93a75a09e531db278c6d2cd25206836c8c2c4071cd7d3118"
+      Computed: "0a5ad2dc13f06f73f96ba94e8d01d48252bc934e2de71a837620ca0fef8a7453"
       Declared: "intentionally bogus value"
     """.trimIndent())
   }

pkl-core/src/test/kotlin/org/pkl/core/project/ProjectTest.kt

@@ -153,7 +153,7 @@ class ProjectTest {
         –– Pkl Error ––
         Cannot download package `package://localhost:12110/fruit@1.0.5` because the computed checksum for package metadata does not match the expected checksum.
         
-        Computed checksum: "b4ea243de781feeab7921227591e6584db5d0673340f30fab2ffe8ad5c9f75f5"
+        Computed checksum: "abd173e8a25f5b930b0e34269a441e32c9d95e0b0a715bc6eff918f0afd0688e"
         Expected checksum: "intentionally bogus checksum"
         Asset URL: "https://localhost:12110/fruit@1.0.5"