PG-2795 install_ppg_tools: one task layout for every major

[dutow]

• pgvector moves up next to the other extensions for 14/15/16, where it sat after etcd together with a second, oddly gated copy of the etcd and haproxy deb install that the unconditional copy right after it made redundant (legacy_layout, patroni_deb_gate)
• pg_tde install follows from major >= 17, pg_oidc_validator from major >= 18 (oidc)
• drop the etcd restart right after its rpm install, nothing configures etcd in between and Start etcd follows later, pg-15 never had it (etcd_restart)
• 17 and 18 gated the versioned pgaudit/postgis rpm names on >= X.0, set them directly instead (vars_gate)
• one set of comments around shared_preload_libraries