schedulers: fix the issue about the limit of the hot region

[nolouch]

Signed-off-by: nolouch [email protected]

What problem does this PR solve?

The auto adjust limit may become 0.

What is changed and how it works?

use 1 as min value.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #1552 into master will decrease coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1552     +/-   ##
=========================================
- Coverage   67.75%   67.56%   -0.2%     
=========================================
  Files         159      159             
  Lines       16141    16138      -3     
=========================================
- Hits        10937    10903     -34     
- Misses       4250     4263     +13     
- Partials      954      972     +18

Impacted Files	Coverage Δ
server/schedulers/hot_region.go	76.66% <100%> (+0.47%)	⬆️
pkg/tempurl/tempurl.go	57.14% <0%> (-14.29%)	⬇️
pkg/metricutil/metricutil.go	90.62% <0%> (-9.38%)	⬇️
pkg/etcdutil/etcdutil.go	86.04% <0%> (-9.31%)	⬇️
server/util.go	70.86% <0%> (-4.64%)	⬇️
server/etcd_kv.go	65.95% <0%> (-4.26%)	⬇️
server/namespace_cluster.go	81.81% <0%> (-3.9%)	⬇️
server/leader.go	80.81% <0%> (-3.68%)	⬇️
server/schedule/region_scatterer.go	86.23% <0%> (-2.18%)	⬇️
server/tso.go	74.46% <0%> (-2.13%)	⬇️
... and 8 more

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