fix(sensitive-issue-searcher): modernise for PHP 8.4

The tool's dependencies were pinned to knplabs/github-api ^2.1 and
cache/redis-adapter ^0.5, both requiring PHP ^5.6 || ^7.0, so composer
install failed on PHP 8.4. The new php-test workflow surfaced this.

- Bump to knplabs/github-api ^3.16 with guzzlehttp/guzzle ^7.13 and
  http-interop/http-factory-guzzle, matching the other tools.
- Drop cache/redis-adapter: it was declared but never used by search.php.
- Adapt search.php to the github-api v3 API:
  - authenticate() no longer takes Client::AUTH_HTTP_TOKEN; use
    AuthMethod::ACCESS_TOKEN.
  - Issue::setPerPage() was removed; pass the page size to ResultPager
    instead.
- Stop tracking vendor/ (it was committed here unlike every other tool;
  .gitignore already ignores it).

Signed-off-by: John Molakvoæ <skjnldsv@protonmail.com>
Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv

sensitive-issue-searcher/composer.json

@@ -1,7 +1,7 @@
 {
     "require": {
-        "knplabs/github-api": "^2.1",
-        "php-http/guzzle6-adapter": "^1.1",
-        "cache/redis-adapter": "^0.5.0"
+        "knplabs/github-api": "^3.16",
+        "guzzlehttp/guzzle": "^7.13",
+        "http-interop/http-factory-guzzle": "^1.2"
     }
 }