fix: do not oversize jit_buffer_size

[am97]

The opcache memory got full on my instance:

The PHP OPcache module is not properly configured. The OPcache buffer is nearly full. To assure that all scripts can be hold in cache, it is recommended to apply "opcache.memory_consumption" to your PHP configuration with a value higher than "128"

It's the same problem that the one raised in #2352 , but a better solution seems to adjust opcache.jit_buffer_size.

This corresponds to the following change in the documentation: nextcloud/documentation@538390d

Single Nextcloud instances have shown to use less than 2 MiB of the configured JIT buffer size, so that 8 MiB is sufficient by a large margin. Using any larger value pre-occupies the opcache.memory_consumption accurdingly and 128 MiB would hence fill its default 128 MiB completely, without any benefit.

[J0WI]

Duplicate of #2185