fix(admin): Revise opcache revalidation tuning section

[joshtrichards]

☑️ Resolves

• Try to make PHP revalidation values, suggestions, and trade-offs more apparent (main focus of this PR)
• Add big warnings about impact of lengthening frequency of revalidations and/or disabling outright
• Try to make the opcache tuning section a bit more organized (move things around more logically, add sub-headings)

Also, related to pending enhancement: nextcloud/server#45498

Note: The main thing that is perhaps controversial here is the PHP default of 2 versus 60 (not so much because we ever formally recommended changing it to 60, but because 60 is already being used in both the AIO and micro-services/community Docker image). I'm of the camp that PHP's default of 2 should be emphasized. And any deviation from that is fine, but the operator making that change is on their own (and expected to restart/etc if needed). Fortunately with the Docker images (at least ours), at least for Server itself, it's less of an issue since the code upgrades only happen through a container restart (but still a factor for config.php + potentially for app updates).

Historical (most relevant):

• Set recommend opcache.revalidate_freq docker#1718
• Use the recommended opcache settings docker#93
• OPcache recommendation wrong (opcache.enable_cli=1) #1439
• adjust opcache settings all-in-one#192
• update opcache settings all-in-one#199

🖼️ Screenshots

[joshtrichards]

/backport to stable29

[joshtrichards]

/backport to stable28

[joshtrichards]

/backport to stable27

[kesselb]

Thanks 👍

I thought about removing validate_timestamp because it appears that many people do not understand the implications.