chore(deps): bump grpc and grpc-tools to >= 1.75

Bump grpc and grpc-tools to >= 1.75. This resolves native extension compilation on macOS:

In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/stdio.h:61:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/_stdio.h:320:7: error: expected ')'
  320 | FILE    *fdopen(int, const char *) __DARWIN_ALIAS_STARTING(__MAC_10_6, __IPHONE_3_2, __DARWIN_EXTSN(fdopen));
      |          ^
third_party/zlib/zutil.h:147:33: note: expanded from macro 'fdopen'
  147 | #        define fdopen(fd,mode) NULL /* No fdopen() */
      |
An error occurred while installing grpc (1.68.1), and Bundler cannot continue.

In Gemfile:
  gitlab-labkit was resolved to 0.42.0.pre.2.pre.g6cb3c9b, which depends on
    grpc

There is only one breaking change between v1.6x and v1.7x: Drop Ruby 3.0 support in v1.74. You can find the full changelog here.

note: native ext requires the C++ compiler with c++17 now.

BREAKING CHANGE: Minimum required version of grpc and grpc-tools is now 1.75.