chore: bump ekka to 0.15.0

[SergeTupchiy]

ekka 0.15.0 uses mria 0.5.0, which adds several fixes, enhancements and features:

• protect mria:join/1,2 with a global lock
• implement new function mria:sync_transaction/4,3,2, which waits for a transaction replication to be ready on the local node (if the local node is a replicant)
• optimize mria:running_nodes/0
• optimize mria:ro_transaction/2 when called on a replicant node.

Fixes: EMQX-9588 (#10380), EMQX-9102, EMQX-9152, EMQX-9213

Summary

🤖 Generated by Copilot at 453b261

Update ekka dependency to enable cluster discovery and autohealing in emqx 5.0.0.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update