All tables should have max 16 indexes

I think we can make a rule that all table should have max 16 indexes. All of the tables in !159609 (comment 2006090899) are likely large, or giant - and will affect both LwLock and WAL. It is unlikely a tiny table will have more than 16 indexes.

• Exceptions can be made so that new indexes can be added but the rollout needs to be monitored.

Here are the top 25 tables with the most indices:

table_name	index count
projects	55
issues	36
merge_requests	32
users	31
ci_builds	26
packages_packages	25
namespaces	25
p_ci_builds	25
ci_pipelines	24
sbom_occurrences	23
vulnerability_reads	23
members	19
deployments	17
vulnerability_occurrences	17
epics	17
vulnerabilities	15
packages_package_files	15
ci_runners	15
ci_job_artifacts	14
snippets	14
approval_merge_request_rules	14
events	14
p_ci_job_artifacts	14