Migrate absl::optional to std::optional in //chrome/browser/ui/webui

- Used sed to replace the files:
  ```
  git sed absl::nullopt std::nullopt
  git sed absl::optional std::optional
  git sed 'third_party/abseil-cpp/absl/types/optional.h' '<optional>'
  ```
- Then used find and replace to fix the imports for <optional> to the
  right location
- Then git cl format

Bug: 1373619
Change-Id: Ic5e887e6002487e251843af6bff2a33d4902c6c7
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5089865


Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#12340436}