Upgrade to Bazel 7.7.0 and protobuf 29.5

[rjhuijsman]

Summary

This PR upgrades the Bazel build to Bazel 7.7.0 as part of a coordinated
upgrade across the entire monorepo. It also upgrades protobuf from 29.3
to 29.5 and glog from 0.5.0 to 0.7.1.

Why things had to change

Bazel 7.7.0 (.bazeliskrc)

Bumped USE_BAZEL_VERSION from 6.5.0 to 7.7.0.

bzlmod disabled (.bazelrc)

Bazel 7 enables bzlmod by default. Added common --enable_bzlmod=false
to keep the existing WORKSPACE-based build working until the project
is ready to migrate to MODULE.bazel.

repo_mapping parameter removed (bazel/repos.bzl, bazel/deps.bzl, picojson/rapidjson)

Bazel 7 removed the repo_mapping parameter from repository rules
(http_archive, git_repository, etc.). All explicit repo_mapping = {}
arguments have been removed from all call sites.

native.existing_rules() deprecated; external parameter removed

Bazel 7 deprecates native.existing_rules(). The pattern of accepting
an external parameter to conditionally register the repo's own
git_repository (so downstream consumers could pull it in) is replaced
throughout with maybe() from @bazel_tools//tools/build_defs/repo:utils.bzl,
which already skips registration if the repo exists.

py_repositories() call added to WORKSPACE.bazel

Protobuf 29.x loads @rules_python//python/py_info.bzl internally at
analysis time. This requires py_repositories() to have been called
first to create the @@rules_python_internal repository. Without this
call the build fails with a cycle in the WORKSPACE loading sequence.

rules_python 1.0.0 added to bazel/repos.bzl

Protobuf 29.x loads rules_python internally; rules_python 1.0.0
must be declared via maybe() before protobuf_deps() runs, or
protobuf_deps() will register an incompatible version.

rules_python 0.25.0 override removed from bazel/deps.bzl

The old deps.bzl called protobuf_deps() and then immediately
overrode rules_python with 0.25.0 to work around a compatibility bug.
With rules_python 1.0.0 now declared via maybe() in repos.bzl
(which runs first), protobuf_deps() will see it already registered
and skip it, making the override unnecessary.

glog 0.5.0 → 0.7.1 (bazel/repos.bzl)

glog 0.5.0 does not compile against protobuf 29.x headers. glog 0.7.1
is the first release with the necessary compatibility fixes.

protobuf 29.3 → 29.5 (bazel/repos.bzl)

Minor bump to align with the rest of the monorepo. The 29.x API changes
required for this repo were already present since the previous 29.3
upgrade.

DynamicCastToGenerated → DynamicCastMessage (include/stout/flags/flags.h)

Protobuf 29.x removed the DynamicCastToGenerated<T>() function from
the C++ runtime. Both call sites are updated to use DynamicCastMessage<T>(),
the direct replacement with identical semantics.

Test plan

• bazel test //... in this repo passes
• bazel test //... in the monorepo passes (consumes this as a submodule)
• PR for dependent repos (pyprotoc-plugin, public, mono) is also merged