[runners-flink] Use index-based version comparison in flink_runner.gradle (#38272)

* [runners-flink] Use index-based version comparison in flink_runner.gradle

`previous_versions` was computed by lexicographic string comparison
against `flink_major`:

    def previous_versions = all_versions.findAll { it < flink_major }

This breaks for `flink_versions=1.17,1.18,1.19,1.20,2.0` (the current
gradle.properties value) because lexicographic ordering disagrees with
semantic ordering whenever a two-digit minor crosses a single-digit
boundary. Concretely, today this resolves to:

    flink_major=1.17 -> []                              (correct)
    flink_major=1.18 -> ["1.17", "1.20"]                (wrong; should be ["1.17"])
    flink_major=1.19 -> ["1.17", "1.18", "1.20"]        (wrong; should be ["1.17", "1.18"])
    flink_major=1.20 -> []                              (wrong; should be ["1.17", "1.18", "1.19"])
    flink_major=2.0  -> ["1.17", "1.18", "1.19", "1.20"](correct)

The 1.18 and 1.19 builds therefore pick up
`runners/flink/1.20/src/main/java/.../DoFnOperator.java` as a "previous"
override, and the 1.20 build loses the 1.19 test overrides
(MemoryStateBackendWrapper, StreamSources).

Replace the lex compare with index lookup against `flink_versions`,
exactly like spark_runner.gradle does (PR #38233). Also:

- Trim whitespace from each entry (defensive against
  `flink_versions=1.17, 1.18, ...`).
- Throw a clear GradleException if `flink_major` isn't listed in
  `flink_versions`, instead of silently producing an empty
  `previous_versions` list.
- Update `use_override` to derive from the same index, mirroring the
  Spark version.

* Trigger Build

* [runners-flink] Address Gemini review on flink_runner.gradle

- Trim/stringify flink_major before indexOf so command-line / GString
  values match the trimmed all_versions entries.
- Drop redundant ternary on previous_versions; subList(0, 0) is already
  empty when flink_major is the first listed version.

* Trigger Build

Author: tkaymak

runners/flink/flink_runner.gradle

@@ -32,9 +32,16 @@ def overrides(versions, type, base_path, group='java') {
   ["${base_path}/src/${type}/${group}"] + versions.collect { "${base_path}/${it}/src/${type}/${group}" } + ["./src/${type}/${group}"]
 }
 
-def all_versions = flink_versions.split(",")
-
-def previous_versions = all_versions.findAll { it < flink_major }
+def all_versions = flink_versions.split(",").collect { it.trim() }
+// Determine version order by list position rather than string comparison so two-digit
+// minors (e.g. "1.20") sort correctly relative to "1.18" / "1.19".
+def flink_major_index = all_versions.indexOf(flink_major?.toString()?.trim())
+if (flink_major_index < 0) {
+  throw new GradleException(
+      "flink_major='${flink_major}' is not listed in flink_versions='${flink_versions}' " +
+      "(see root gradle.properties).")
+}
+def previous_versions = all_versions.subList(0, flink_major_index)
 
 // Version specific code overrides.
 def main_source_overrides = overrides(previous_versions, "main", base_path)
@@ -106,7 +113,7 @@ def copyTestResourcesOverrides = tasks.register('copyTestResourcesOverrides', Co
   it.duplicatesStrategy DuplicatesStrategy.INCLUDE
 }
 
-def use_override = (flink_major != all_versions.first())
+def use_override = (flink_major_index > 0)
 def sourceBase = "${project.projectDir}/../src"
 
 if (use_override) {