Release: add REL_2_STABLE branch support to CI

[tuhaihe]

• Add REL_2_STABLE branch to pxf-ci.yml and dependency-submission.yml workflows
• Update CLOUDBERRY_VERSION environment variable to REL_2_STABLE

This change enables testing and dependency submission for the REL_2_STABLE release branch.

closes: #ISSUE_Number

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[tuhaihe]

Support testing PXF REL_2_STABLE branch on Cloudberry REL_2_STABLE.

[ostinru]

LGTM