Issue #14877: drop java 14 build

[strkkk]

Closes #14877

~/IdeaProjects/checkstyle (14877_drop_java14)
$ grep -Rl --include='*.java' ': Compilable with Java14' | wc -l
0

~/IdeaProjects/checkstyle (14877_drop_java14)
$ grep -Rl --include='*.java' ': Compilable with Java17' | wc -l
242

[romani]

Item

[romani]

ok to merge.

My confusion was on line where javac error.

src/test/resources-noncompilable/com/puppycrawl/tools/checkstyle/checks/coding/equalsavoidnull/InputEqualsAvoidNullEnhancedInstanceof.java:13: error: expression type String is a subtype of pattern type String
        if (str instanceof String myString) {
                ^

I still cannot expalin why 206cb0d
is passing
https://app.circleci.com/pipelines/github/checkstyle/checkstyle/25600/workflows/eab6c959-c9a9-44a1-a34b-5b3d0bcdea67/jobs/593792
but it is ok, may be I am blind.