[fix][doc] Fix links to find older PIPs (apache#20882)

Author: asafm

pip/README.md

@@ -65,10 +65,10 @@ A PIP proposal can be in these states:
 The process works in the following way:
 
 1. Fork https://github.com/apache/pulsar repository (Using the fork button on GitHub).
-2. Clone the repository, and on it, copy the file `pip/TEMPLATE.md` and name it `pip-xxx.md`. The number `xxx` should be the next sequential number after the last contributed PIP. You view the list of contributed PIPs (at any status) as a list of Pull Requests having a title starting with `[pip][design] PIP-`. Use the link [here](https://github.com/apache/pulsar/pulls?q=is%3Apr+title%3A%22%5Bpip%5D%5Bdesign%5D+PIP-%22) as shortcut.
+2. Clone the repository, and on it, copy the file `pip/TEMPLATE.md` and name it `pip-xxx.md`. The number `xxx` should be the next sequential number after the last contributed PIP. You view the list of contributed PIPs (at any status) as a list of Pull Requests having a "PIP" label. Use the link [here](https://github.com/apache/pulsar/pulls?q=is%3Apr+label%3APIP+) as shortcut.
 3. Write the proposal following the section outlined by the template and the explanation for each section in the comment it contains (you can delete the comment once done).
    * If you need diagrams, avoid attaching large files. You can use [MermaidJS](https://mermaid.js.org/) as simple language to describe many types of diagrams. 
-4. Create GitHub Pull request (PR). The PR title should be `[pip][design] PIP-xxx: {title}`, where the `xxx` match the number given in previous step (file-name). Replace `{title}` with a short title to your proposal.
+4. Create GitHub Pull request (PR). The PR title should be `[improve][pip] PIP-xxx: {title}`, where the `xxx` match the number given in previous step (file-name). Replace `{title}` with a short title to your proposal.
 5. The author(s) will email the [email protected] mailing list to kick off a discussion, using subject prefix `[DISCUSS] PIP-xxx: {PIP TITLE}`. The discussion will happen in broader context either on the mailing list or as general comments on the PR. Many of the discussion items will be on particular aspect of the proposal, hence they should be as comments in the PR to specific lines in the proposal file.
 6. Update file with a link to the discussion on the mailing. You can obtain it from [Apache Pony Mail](https://lists.apache.org/[email protected]).
 7. Based on the discussion and feedback, some changes might be applied by authors to the text of the proposal. They will be applied as extra commits, making it easier to track the changes.