docs: remove dead Gitter badge and link from README

[ssmurfgg04-gif]

Describe your change:

Removed the dead Gitter badge and the Gitter mention from the README, keeping the Discord link as the community channel. (This replaces #15211, which the bot closed before the checklist was filled.)

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

Details: https://gitter.im/TheAlgorithms/community returns 404 (Gitter shut down its community pages). The README currently shows a broken Gitter badge and points people to it under "Community Channels". Discord remains live and is already linked, so the Gitter references are removed.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one file (README.md). To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory. (n/a: no Python files changed)
• All filenames are in all lowercase characters with no spaces or dashes. (n/a: no new files)
• All functions and variable names follow Python naming conventions. (n/a: no code changes)
• All function parameters and return values are annotated with Python type hints. (n/a: no code changes)
• All functions have doctests that pass the automated testing. (n/a: no code changes)
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. (n/a: no new algorithms)
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword. (n/a: no open issue for this)

Items not marked are not applicable: this PR only edits README.md (documentation).