server: change log level for the stale region when loading

[rleungx]

What problem does this PR solve?

Related to #2427, but for release-3.0

What is changed and how it works?

This PR changes the log level of the stale region when loading.

Check List

Tests

• Unit test

Release note

• Change the log level of the stale region when loading regions

[codecov]

Codecov Report

Merging #3064 into release-3.0 will decrease coverage by 2.90%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-3.0    #3064      +/-   ##
===============================================
- Coverage        69.92%   67.02%   -2.91%     
===============================================
  Files              171      171              
  Lines            16980    14333    -2647     
===============================================
- Hits             11873     9606    -2267     
+ Misses            4138     3770     -368     
+ Partials           969      957      -12     

Impacted Files	Coverage Δ
server/cluster_info.go	86.76% <100.00%> (-1.63%)	⬇️
pkg/etcdutil/etcdutil.go	83.33% <0.00%> (-12.02%)	⬇️
server/schedule/scheduler.go	55.55% <0.00%> (-11.12%)	⬇️
server/schedulers/base_scheduler.go	53.33% <0.00%> (-9.83%)	⬇️
server/util.go	65.67% <0.00%> (-9.49%)	⬇️
server/tso.go	74.41% <0.00%> (-9.09%)	⬇️
server/api/cluster.go	66.66% <0.00%> (-8.34%)	⬇️
tools/pd-ctl/pdctl/command/cluster_command.go	66.66% <0.00%> (-8.34%)	⬇️
server/namespace/namespace.go	36.84% <0.00%> (-8.32%)	⬇️
...ls/pd-ctl/pdctl/command/table_namespace_command.go	52.17% <0.00%> (-7.83%)	⬇️
... and 162 more

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[HunDunDM]

LGTM

[HunDunDM]

/run-all-tests