[theme] Replace legacy theme.palette.text.hint with secondary

[dmtrKovalenko]

• I have followed (at least) the PR section of the contributing guide.

Closes #1680
Closes #1681

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/mui-org/material-ui-pickers/gjkyxa5dq
✅ Preview: https://material-ui-pickers-git-feature-use-themepalettetextsecondary.mui-org.vercel.app

[cypress]

---

Test summary

75 • 0 • 3 • 0

---

Run details

Project	material-ui-pickers
Status	Passed
Commit	2ecb62e
Started	Jul 3, 2020 9:10 AM
Ended	Jul 3, 2020 9:11 AM
Duration	01:37 💡
OS	Linux Debian - 10.0
Browser	Chrome 80

View run in Cypress Dashboard ➡️

---

This comment has been generated by cypress-bot as a result of this project's GitHub integration settings. You can manage this integration in this project's settings in the Cypress Dashboard

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (next@510c745). Click here to learn what that means.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             next    #1952   +/-   ##
=======================================
  Coverage        ?   91.34%           
=======================================
  Files           ?       84           
  Lines           ?     2473           
  Branches        ?      468           
=======================================
  Hits            ?     2259           
  Misses          ?      174           
  Partials        ?       40           

Impacted Files	Coverage Δ
lib/src/views/Calendar/Calendar.tsx	86.76% <ø> (ø)
lib/src/views/Calendar/Day.tsx	92.85% <ø> (ø)
lib/src/views/Calendar/Month.tsx	100.00% <ø> (ø)
lib/src/views/Calendar/Year.tsx	96.87% <ø> (ø)
lib/src/views/Clock/ClockNumber.tsx	97.22% <100.00%> (ø)
lib/src/Picker/Picker.tsx	93.75% <0.00%> (ø)
lib/src/views/Calendar/CalendarView.tsx	100.00% <0.00%> (ø)
lib/src/TimePicker/TimePickerToolbar.tsx	94.64% <0.00%> (ø)
lib/src/_shared/ArrowSwitcher.tsx	88.88% <0.00%> (ø)
lib/src/DateTimePicker/DateTimePicker.tsx	97.87% <0.00%> (ø)
... and 79 more

---

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 510c745...2ecb62e. Read the comment docs.