[TRIVIAL] chore: Default validator-keys-tool to build from master branch

High Level Overview of Change

master is the default branch for that project. There's no point in using develop.

Context of Change

https://github.com/ripple/validator-keys-tool/pull/40#issuecomment-1984374109

I just rebased develop onto master. After resolving conflicts, the branches were identical.

If there are no objections, I'm going to close this PR and force push that updated develop.

I think the correct solution is to update the check in rippled to default to use the master branch, and delete this develop branch entirely.

Type of Change

[X ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)

Before / After

These changes only matter when building https://github.com/ripple/validator-keys-tool from within the rippled repo. e.g.

git checkout develop
cmake -Dvalidator-keys=ON [...] <build dir>
cmake --build <build dir> --target validator-keys

Before:

If building from any rippled branch other than master or release, validator-keys will be built from its develop branch.

After:

If building from any rippled branch other than release, validator-keys will be built from its master branch.

Future Tasks

If this PR is merged, the develop branch can be deleted from https://github.com/ripple/validator-keys-tool.

XRPLF / rippled

[TRIVIAL] chore: Default validator-keys-tool to build from master branch #4943