Set --combined_report to lcov by default

It's very likely that someone running `bazel coverage` wants a combined report and the message announcing the report's path makes the effect of `bazel coverage` easier to understand.

RELNOTES[INC]: `--combined_report` now defaults to `lcov` (was: `none`).

Closes #26328.

PiperOrigin-RevId: 772916208
Change-Id: I791313fadda5360c82882c1830d12f98d7755e1c

Author: fmeum

src/main/java/com/google/devtools/build/lib/bazel/coverage/BazelCoverageReportModule.java

@@ -51,7 +51,7 @@ public static class Options extends OptionsBase {
         converter = ReportTypeConverter.class,
         documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
         effectTags = {OptionEffectTag.UNKNOWN},
-        defaultValue = "none",
+        defaultValue = "lcov",
         help =
             "Specifies desired cumulative coverage report type. At this point only LCOV "
                 + "is supported.")