feat!: drops support of Java 7

[thiagotnunes]

From the next major release we will be only supporting Java 8+.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/workflows/ci.yaml
• .kokoro/nightly/java7.cfg
• .kokoro/presubmit/java7.cfg
• CONTRIBUTING.md

[thiagotnunes]

This should only be merged when we are planning the next major bump (for CMEK, #666).

[codecov]

Codecov Report

Merging #946 (a3bece7) into master (32fdd60) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #946      +/-   ##
============================================
- Coverage     85.24%   85.21%   -0.03%     
+ Complexity     2660     2570      -90     
============================================
  Files           146      146              
  Lines         14404    14159     -245     
  Branches       1395     1313      -82     
============================================
- Hits          12278    12066     -212     
- Misses         1542     1544       +2     
+ Partials        584      549      -35     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/spanner/DatabaseClientImpl.java	76.69% <0.00%> (-7.77%)	21.00% <0.00%> (ø%)
.../cloud/spanner/spi/v1/SpannerErrorInterceptor.java	52.94% <0.00%> (-7.06%)	3.00% <0.00%> (ø%)
...ud/spanner/connection/ClientSideStatementImpl.java	48.88% <0.00%> (-1.12%)	9.00% <0.00%> (ø%)
...oogle/cloud/spanner/PartitionedDmlTransaction.java	82.02% <0.00%> (-0.59%)	14.00% <0.00%> (-1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	88.73% <0.00%> (-0.53%)	71.00% <0.00%> (-2.00%)
...m/google/cloud/spanner/spi/v1/GapicSpannerRpc.java	81.41% <0.00%> (-0.48%)	71.00% <0.00%> (-12.00%)
...e/cloud/spanner/connection/CredentialsService.java	90.00% <0.00%> (-0.48%)	8.00% <0.00%> (ø%)
...m/google/cloud/spanner/TransactionManagerImpl.java	87.03% <0.00%> (-0.47%)	21.00% <0.00%> (ø%)
.../com/google/cloud/spanner/AbstractReadContext.java	86.54% <0.00%> (-0.33%)	42.00% <0.00%> (-6.00%)
.../admin/instance/v1/stub/GrpcInstanceAdminStub.java	96.94% <0.00%> (-0.24%)	21.00% <0.00%> (-1.00%)
... and 23 more

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