[improve][misc] Deprecate PIP issue type

[asafm]

PIP: #20207

Motivation

We've switched to PRs for the process of submitting PIPs.

Modifications

• Altered the description of PIP GitHub issue type, so people will stop using it and use a GitHub PR
• Fix the link to the new location of the PIP process

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[tisonkun]

Merging...

This is an accepted proposal.