README: change release icon always show the latest tag

[crazycs520]

What problem does this PR solve?

Make README Always show the latest(largest) tag version. Currently README will always get the latest-published version.
change

to

What is changed and how it works?

change
https://img.shields.io/github/release/pingcap/tidb.svg
to
https://img.shields.io/github/tag/pingcap/tidb.svg?label=release

Check List

Tests

• No Code

[winoros]

lgtm

[codecov-io]

Codecov Report

Merging #8974 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8974      +/-   ##
==========================================
+ Coverage    67.5%   67.52%   +0.01%     
==========================================
  Files         363      363              
  Lines       75216    75216              
==========================================
+ Hits        50777    50789      +12     
+ Misses      19950    19942       -8     
+ Partials     4489     4485       -4

Impacted Files	Coverage Δ
store/tikv/lock_resolver.go	41.7% <0%> (-0.95%)	⬇️
executor/join.go	76.27% <0%> (ø)	⬆️
executor/distsql.go	73% <0%> (+0.46%)	⬆️
util/filesort/filesort.go	76.48% <0%> (+0.94%)	⬆️
ddl/delete_range.go	74.28% <0%> (+1.71%)	⬆️
ddl/session_pool.go	93.1% <0%> (+10.34%)	⬆️
util/systimemon/systime_mon.go	100% <0%> (+20%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 884db0a...984e89a. Read the comment docs.

[shenli]

LGTM