feat(github): add a stale bot for PRs

[aeddi]

Closes https://github.com/gnolang/gno/issues/1445

Contributors' checklist...

• [x] Added new tests, or not needed, or not feasible
• [x] Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• [x] Updated the official documentation or not needed
• [x] No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• [x] Added references to related issues and PRs
• [ ] Provided any useful hints for running manual tests
• [ ] Added new benchmarks to generated graphs, if any. More info here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.00%. Comparing base (f2928f1) to head (6b353a6). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2804      +/-   ##
==========================================
- Coverage   63.35%   63.00%   -0.35%     
==========================================
  Files         548      548              
  Lines       78646    81330    +2684     
==========================================
+ Hits        49825    51245    +1420     
- Misses      25461    26653    +1192     
- Partials     3360     3432      +72     

Flag	Coverage Δ
contribs/gnodev	61.11% <ø> (ø)
contribs/gnofaucet	14.82% <ø> (-0.95%)	:arrow_down:
gno.land	67.18% <ø> (ø)
gnovm	67.88% <ø> (ø)
misc/genstd	79.72% <ø> (ø)
tm2	62.44% <ø> (+<0.01%)	:arrow_up:

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[aeddi]

Applied the changes you asked me @moul during our one-on-one chat.

[thehowl]

@Kouteki to be aware of the rules.

I'd say after exempting milestones, we're good to try and test this out. There may be eventually more fine-grained rules to add, or we can simply get around with the milestone exemption. Excited to see this working.

[Kouteki]

Relates to https://github.com/gnolang/gno/issues/3072