syz-manager: per-syscall limits too restrictive

**Describe the bug**
`minimizeCorpusLocked()` in `syz-manager/manager.go` contains hardcoded limits for rejecting corpus entries if a given syscall occurs too frequently in the corpus. According to the code comments, this is a defensive mechanism against "corpus explosions".
We think the current implementation of this heuristic actively holds back syzkaller performance on `io_uring` related syscalls on Linux. By design, `io_uring` contains a lot of complexity and code diversity behind few individual real and artificial syzkaller syscalls.

Affected:
* `io_uring_setup()`
* `io_uring_enter()`
* `syz_io_uring_setup()`
* `syz_io_uring_submit()`

The `io_uring_register()` syscall is potentially less affected, since it has explicit sub-variants (`io_uring_register$ABC`).

Note that this problem gets worse over time as more functionality is reachable via `io_uring` on newer kernels and expressed in syzlang grammar, because more unique corpus entries will call into the same syscalls.

**To Reproduce**
During local syzkaller fuzzing, we ran into the "coverage for X has saturated, not accepting more inputs" warning that signals rejections by this heuristic, defined [here](https://github.com/google/syzkaller/blob/1c2ff3f985f46c14d19fdadaf02516787d3f5449/syz-manager/manager.go#L1143C16-L1143C72).

**Expected behavior**
Fuzzer behavior on complex syscalls is not excessively restricted, or known complex syscalls are allowlisted into a second tier of restrictions.

**Additional context**
Issue observed during an X41 D-Sec GmbH research project into io_uring Linux kernel security, sponsored by OSTIF.

Start in syz-manager/manager.go at minimizeCorpusLocked(), especially the hardcoded limits linked near line 1143. Reproduce the "coverage for X has saturated, not accepting more inputs" warning during local syzkaller fuzzing and compare behavior for the listed io_uring syscalls. Done means complex syscalls are no longer excessively restricted while the corpus-explosion defense remains effective.

Written by the indexing model from the issue text.