CI: pin Cloudberry branch to REL_2_STABLE

[tuhaihe]

Switch the CI Cloudberry checkout from main to REL_2_STABLE branch.

Cloudberry main has upgraded to the PG16 kernel, whose tightened role grant permission checks (GRANT ... GRANTED BY now requires ADMIN OPTION) break multiple backup integration tests. REL_2_STABLE still runs PG14 and passes the full test suite without these failures.

Add a TODO to revert to main once Cloudberry 3.x stabilizes with PG16.

See: #103

closes: #ISSUE_Number

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[tuhaihe]

In cloudberry-pxf, we've now moved the main branch to REL_2_STABLE: apache/cloudberry-pxf@d6eba0d

[tuhaihe]

cc @leborchuk @woblerr Love to hear your thoughts. Thanks!

[leborchuk]

LGTM, now the stable branch is REL_2_STABLE