Log warning message when no Github API token is specified

[doodlesbykumbi]

Desired Outcome

A warning serves the dual purpose of informing the user of the possibility of specifying the Github API token, and anticipating a likely failure mode

Example logs:

@doodlesbykumbi ➜ /workspaces/conjur-oss-suite-release (main ✗) $ ./parse-changelogs -v v1.14.1+suite.1 -t release -o RELEASE_NOTES.md
2021/11/15 21:10:19 Starting changelog parser...
2021/11/15 21:10:19 Parsing linked repositories...
2021/11/15 21:10:19 Reading suite.yml...
2021/11/15 21:10:19 Unmarshaling data...
2021/11/15 21:10:19 Releases dir: releases
2021/11/15 21:10:19 Using releases/suite_1.13.1+suite.1.yml as previous release for pinning
2021/11/15 21:10:19 Reading releases/suite_1.13.1+suite.1.yml...
2021/11/15 21:10:19 Unmarshaling data...
2021/11/15 21:10:19 Collecting changelogs...
2021/11/15 21:10:19 WARN: No Github API token specified (via "-p" flag or "GITHUB_TOKEN" environment variable). This run might FAIL due to the API rate limit
2021/11/15 21:10:19 Processing category: Conjur Server
2021/11/15 21:10:19 - Processing repo: cyberark/conjur
2021/11/15 21:10:19   Fetching https://api.github.com/repos/cyberark/conjur/releases...
2021/11/15 21:10:19 ERROR: code 403: https://api.github.com/repos/cyberark/conjur/releases: {"message":"API rate limit exceeded for 51.124.226.40. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}
exit status 1

Connected Issue/Story

Resolves -

CyberArk internal issue link: ONYX-14071

Changelog

• [ ] The CHANGELOG has been updated, or
• [x] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] This PR includes new unit and integration tests to go with the code changes, or
• [x] The changes in this PR do not require tests

Documentation

• [ ] Docs (e.g. READMEs) were updated in this PR
• [ ] A follow-up issue to update official docs has been filed here: [insert issue ID]()
• [x] This PR does not require updating any documentation

Behavior

N/A

• [ ] This PR changes product behavior and has been reviewed by a PO, or
• [ ] These changes are part of a larger initiative that will be reviewed later, or
• [ ] No behavior was changed with this PR

Security

• [ ] Security architect has reviewed the changes in this PR,
• [ ] These changes are part of a larger initiative with a separate security review, or
• [x] There are no security aspects to these changes