Update FTL %cpu and %mem everytime total CPU stats are updated

[yubiuser]

What does this PR aim to accomplish?:

Currently, the cpu and mem stats for FTL are less frequently updated than the total cpu and mem stats on the settings/system page. Reason is, that FTL stats are refreshed only every 2 minutes (REFRESH_INTERVAL.ftl), but total cpu and mem every 20 seconds ( REFRESH_INTERVAL.system).

This PR builds upon pi-hole/FTL#2645 which provides FTL's CPU and mem share within the /info/system endpoint as well.

How does this PR accomplish the above?:

Move the update function from updateFtlInfo() to updateSystemInfo()

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[yubiuser]

Second commit adds

[DL6ER]

Merge only when the FTL PR is merged, too

[rdwebdesign]

Actually this PR can be merged any time.
It relies on the already merged PR pi-hole/FTL#2645.

The other FTL PR just improves the previous one.