Updated issue templates(formatting and rustbot label reference)

[xFrednet]

This PR updates our issue templates. The changes are:

1. Make the Meta section smaller and not indented

   The current format sometimes gets messed up when a user simply pasts the rustc version into the issue without indenting it to match the code block. Removing the indention should hopefully help with formatting in the future.

   Example of messed up formatting

   

   Additionally, I've removed the Version of Clippy from the template. Every issue that I have seen had matching rustc and Clippy versions, and just asking for rustc -Vv makes the issue look cleaner IMO.

2. Add a comment explaining how additional labels can be added with @rustbot label +<label>. The explanation looks like this:

   <!--
   Additional labels can be added to this issue by including the following command:
   
   @rustbot label +<label>
   
   Common labels for this issue type are:
   * `I-suggestion-causes-error`
   -->
   

   The example @rustbot call can sadly not be marked as code inside a comment block. But the example will not cause the bot to add any labels. See Test issue (Testing the label command from rustbot) #7599

---

changelog: none

cc: @rust-lang/clippy

[rust-highfive]

r? @camsteffen

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[camsteffen]

Thanks! @bors r+

[bors]

📌 Commit 10909ae has been approved by camsteffen

[bors]

⌛ Testing commit 10909ae with merge 5e1e9b0...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: camsteffen
Pushing 5e1e9b0 to master...