chore(ekka): Bump version to 0.15.8 (mria 0.5.10)

[ieQu1]

Fixes

Summary

🤖 Generated by Copilot at 1df7499

Updated ekka dependency to version 0.15.8 in both rebar.config and mix.exs files. This fixed a cluster autoheal bug and improved emqx clustering stability.

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

[coveralls]

Pull Request Test Coverage Report for Build 5624525489

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 29 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.01%) to 81.44%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_dashboard/src/emqx_dashboard_monitor.erl	1	65.52%
apps/emqx_ft/src/emqx_ft_storage_exporter_fs.erl	1	80.43%
apps/emqx_gateway_exproto/src/emqx_exproto_gcli.erl	1	64.15%
apps/emqx_gateway/src/emqx_gateway_schema.erl	1	94.44%
apps/emqx_management/src/emqx_mgmt_util.erl	1	13.79%
apps/emqx/src/emqx_channel.erl	2	87.12%
apps/emqx/src/emqx_trace/emqx_trace.erl	2	88.18%
apps/emqx_oracle/src/emqx_oracle.erl	4	79.75%
apps/emqx/src/emqx_reason_codes.erl	16	85.29%

Totals
Change from base Build 5622602750:	0.01%
Covered Lines:	30965
Relevant Lines:	38022

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💛 - Coveralls